And another:

[ 3088.350638] ------------[ cut here ]------------
[ 3088.350639] kernel BUG at ../fs/btrfs/relocation.c:1449!
[ 3088.350643] invalid opcode: 0000 [#1] SMP PTI
[ 3088.350647] CPU: 2 PID: 5222 Comm: btrfs Tainted: G           O     4.12.14-lp150.12.58-default #1 openSUSE Leap 15.0
[ 3088.350648] Hardware name: Dell Inc. OptiPlex 9020/00V62H, BIOS A24 10/24/2018
[ 3088.350650] task: ffff880398564000 task.stack: ffffc9000b2f8000
[ 3088.350671] RIP: 0010:create_reloc_root+0x1dc/0x1f0 [btrfs]
[ 3088.350673] RSP: 0018:ffffc9000b2fb858 EFLAGS: 00010282
[ 3088.350675] RAX: 00000000ffffffef RBX: ffff880409763000 RCX: 0000000000000001
[ 3088.350677] RDX: 0000000000000027 RSI: ffff8803a90dc380 RDI: 0000000000000286
[ 3088.350678] RBP: ffff8803a07209d8 R08: ffff8803a13afa80 R09: 0000000000000000
[ 3088.350679] R10: 0000000000000002 R11: ffff8803a90dc380 R12: fffffffffffffff7
[ 3088.350681] R13: ffff880370b5c000 R14: ffff8803689f0800 R15: 00000000000c4000
[ 3088.350683] FS:  00007f1d8bfb98c0(0000) GS:ffff88041ea80000(0000) knlGS:0000000000000000
[ 3088.350684] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3088.350685] CR2: 00007f33879f7000 CR3: 00000003ab59e005 CR4: 00000000001606e0
[ 3088.350687] Call Trace:
[ 3088.350700]  btrfs_init_reloc_root+0x5b/0xa0 [btrfs]
[ 3088.350710]  record_root_in_trans+0xb7/0xf0 [btrfs]
[ 3088.350720]  btrfs_record_root_in_trans+0x4e/0x60 [btrfs]
[ 3088.350729]  start_transaction+0xa6/0x410 [btrfs]
[ 3088.350738]  __btrfs_prealloc_file_range+0xbb/0x460 [btrfs]
[ 3088.350748]  btrfs_prealloc_file_range+0x10/0x20 [btrfs]
[ 3088.350759]  prealloc_file_extent_cluster+0x113/0x200 [btrfs]
[ 3088.350769]  relocate_file_extent_cluster+0x8d/0x470 [btrfs]
[ 3088.350778]  ? __btrfs_end_transaction+0x1c1/0x2e0 [btrfs]
[ 3088.350787]  relocate_data_extent+0x78/0xc0 [btrfs]
[ 3088.350796]  relocate_block_group+0x495/0x6f0 [btrfs]
[ 3088.350805]  btrfs_relocate_block_group+0x188/0x230 [btrfs]
[ 3088.350815]  btrfs_relocate_chunk+0x4a/0xf0 [btrfs]
[ 3088.350825]  __btrfs_balance+0x8d3/0xbe0 [btrfs]
[ 3088.350835]  btrfs_balance+0x2de/0x5c0 [btrfs]
[ 3088.350844]  btrfs_ioctl_balance+0x310/0x370 [btrfs]
[ 3088.350848]  ? __switch_to_asm+0x40/0x70
[ 3088.350857]  btrfs_ioctl+0xba5/0x1e60 [btrfs]
[ 3088.350860]  ? __switch_to_asm+0x40/0x70
[ 3088.350861]  ? __switch_to_asm+0x34/0x70
[ 3088.350863]  ? __switch_to_asm+0x40/0x70
[ 3088.350865]  ? __switch_to_asm+0x34/0x70
[ 3088.350867]  ? __switch_to_asm+0x40/0x70
[ 3088.350875]  ? __switch_to_asm+0x34/0x70
[ 3088.350882]  ? __switch_to_asm+0x40/0x70
[ 3088.350890]  ? __switch_to_asm+0x34/0x70
[ 3088.350897]  ? __switch_to_asm+0x40/0x70
[ 3088.350904]  ? __switch_to_asm+0x34/0x70
[ 3088.350912]  ? __switch_to_asm+0x40/0x70
[ 3088.350919]  ? __switch_to_asm+0x34/0x70
[ 3088.350926]  ? __switch_to_asm+0x40/0x70
[ 3088.350934]  ? __switch_to_asm+0x34/0x70
[ 3088.350942]  ? do_vfs_ioctl+0x90/0x5f0
[ 3088.350956]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
[ 3088.350963]  do_vfs_ioctl+0x90/0x5f0
[ 3088.350970]  ? __schedule+0x247/0x860
[ 3088.350978]  SyS_ioctl+0x74/0x80
[ 3088.350986]  do_syscall_64+0x7b/0x150
[ 3088.350994]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 3088.351001] RIP: 0033:0x7f1d8b0594a7
[ 3088.351008] RSP: 002b:00007ffcbc330b48 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
[ 3088.351015] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f1d8b0594a7
[ 3088.351022] RDX: 00007ffcbc330be0 RSI: 00000000c4009420 RDI: 0000000000000003
[ 3088.351029] RBP: 0000000000000003 R08: 00007f1d8bfb98c0 R09: 00007f1d8bfb98c0
[ 3088.351035] R10: 000000000000055b R11: 0000000000000206 R12: 00007ffcbc330be0
[ 3088.351042] R13: 00005630119bb88e R14: 00007f1d8b31f808 R15: 00007ffcbc3335e8
[ 3088.351049] Code: 48 c7 83 dc 00 00 00 00 00 00 00 48 c7 83 e4 00 00 00 00 00 00 00 c6 83 ec 00 00 00 00 c6 83 ed 00 00 00 00 e9 1b ff ff ff 0f 0b <0f> 0b 0f 0b 0f 0b 0f 0b 66 66 2e 0f 1f 84 00 00 00 00 00 90 0f
[ 3088.351072] Modules linked in: usblp ccm af_packet ebtable_filter ebtables nf_log_ipv6 devlink xt_comment nf_log_ipv4 nf_log_common xt_LOG xt_limit nfnetlink_cthelper nfnetlink vboxpci(O) vboxnetadp(O) vboxnetflt(O) ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT xt_pkttype xt_tcpudp iptable_filter vboxdrv(O) ip6table_mangle nf_conntrack_netbios_ns nf_conntrack_broadcast nf_conntrack_ipv4 nf_defrag_ipv4 ip_tables xt_conntrack bnep nf_conntrack libcrc32c ip6table_filter ip6_tables x_tables msr nls_iso8859_1 nls_cp437 vfat fat snd_hda_codec_hdmi arc4 intel_rapl snd_hda_codec_realtek snd_hda_codec_generic x86_pkg_temp_thermal intel_powerclamp coretemp iwlmvm kvm_intel btusb mac80211 btrtl kvm snd_hda_intel btbcm irqbypass btintel snd_hda_codec iwlwifi crc32_pclmul ghash_clmulni_intel snd_hda_core
[ 3088.351122]  bluetooth dell_wmi snd_hwdep dell_smbios pcbc ecdh_generic cfg80211 mei_wdt aesni_intel sparse_keymap dcdbas aes_x86_64 crypto_simd glue_helper cryptd rfkill i2c_i801 snd_pcm pcspkr e1000e mei_me lpc_ich ptp mei shpchp pps_core snd_timer snd thermal wmi soundcore button btrfs xor hid_generic usbhid raid6_pq sr_mod cdrom i915 i2c_algo_bit drm_kms_helper xhci_pci syscopyarea ehci_pci sysfillrect sysimgblt xhci_hcd ahci fb_sys_fops ehci_hcd crc32c_intel libahci drm usbcore drm_panel_orientation_quirks video sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua efivarfs
[ 3088.351166] ---[ end trace 52c5aab9c3cd8f5e ]---
[ 3088.351181] RIP: 0010:create_reloc_root+0x1dc/0x1f0 [btrfs]
[ 3088.351187] RSP: 0018:ffffc9000b2fb858 EFLAGS: 00010282
[ 3088.351194] RAX: 00000000ffffffef RBX: ffff880409763000 RCX: 0000000000000001
[ 3088.351201] RDX: 0000000000000027 RSI: ffff8803a90dc380 RDI: 0000000000000286
[ 3088.351208] RBP: ffff8803a07209d8 R08: ffff8803a13afa80 R09: 0000000000000000
[ 3088.351215] R10: 0000000000000002 R11: ffff8803a90dc380 R12: fffffffffffffff7
[ 3088.351221] R13: ffff880370b5c000 R14: ffff8803689f0800 R15: 00000000000c4000
[ 3088.351229] FS:  00007f1d8bfb98c0(0000) GS:ffff88041ea80000(0000) knlGS:0000000000000000
[ 3088.351236] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3088.351242] CR2: 00007f33879f7000 CR3: 00000003ab59e005 CR4: 00000000001606e0

I got exactly the same dump while the boot process and then the VM stopped working.

Here is the guide how I could fix it after hours:

https://forums.suse.com/showthread.php?13662-Server-crashes-with-a-long-BTRFS-error-list&p=57729#post57729

I just ran into this on a btrfs volume on OpenSUSE 15.1 (4.12.14-lp151.28.4).  I was unable to ever successfully balance it but mounting with skip_balance at least let me mount it and back everything up.  Here's the gory details:

[  352.337489] BTRFS info (device dm-4): use lzo compression
[  352.338112] BTRFS info (device dm-4): disk space caching is enabled
[  352.338649] BTRFS info (device dm-4): has skinny extents
[  352.348289] BTRFS info (device dm-4): bdev /dev/mapper/slow-Slow errs: wr 0, rd 0, flush 0, corrupt 187, gen 0
[  352.855165] BTRFS info (device dm-4): detected SSD devices, enabling SSD mode
[  352.906205] BTRFS info (device dm-4): checking UUID tree
[  352.906324] BTRFS info (device dm-4): continuing balance
[  352.940505] BTRFS info (device dm-4): relocating block group 2355407880192 flags data
[  352.967918] ------------[ cut here ]------------
[  352.969550] kernel BUG at ../fs/btrfs/relocation.c:1449!
[  352.971205] invalid opcode: 0000 [#1] SMP PTI
[  352.972840] CPU: 2 PID: 4951 Comm: btrfs-balance Tainted: P           O     4.12.14-lp151.28.4-default #1 openSUSE Leap 15.1
[  352.974552] Hardware name: Dell Inc. Precision 7510/0M91XC, BIOS 1.16.3 09/12/2018
[  352.976153] task: ffff880fcb094080 task.stack: ffffc9000a7bc000
[  352.977429] RIP: 0010:create_reloc_root+0x1dc/0x1f0 [btrfs]
[  352.978675] RSP: 0018:ffffc9000a7bf9d0 EFLAGS: 00010282
[  352.979938] RAX: 00000000ffffffef RBX: ffff880ff06daa00 RCX: ffffea0040a6795f
[  352.981133] RDX: 0000000000000013 RSI: ffff880dfa0d8f50 RDI: 0000000000000286
[  352.982124] RBP: ffff880faac771e0 R08: ffff880e0415cea0 R09: 0000000000000000
[  352.983115] R10: 0000000000000028 R11: ffff880dfa0d8f50 R12: fffffffffffffff7
[  352.984098] R13: ffff880feb532000 R14: ffff880fefbdb800 R15: 00000000000c4000
[  352.985072] FS:  0000000000000000(0000) GS:ffff881082500000(0000) knlGS:0000000000000000
[  352.986006] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  352.986782] CR2: 000055f50cacc470 CR3: 000000000200a004 CR4: 00000000003606e0
[  352.987559] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  352.988311] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  352.989043] Call Trace:
[  352.989763]  btrfs_init_reloc_root+0x5b/0xa0 [btrfs]
[  352.990473]  record_root_in_trans+0xb4/0xe0 [btrfs]
[  352.991132]  btrfs_record_root_in_trans+0x4e/0x60 [btrfs]
[  352.991689]  start_transaction+0xa6/0x410 [btrfs]
[  352.992259]  __btrfs_prealloc_file_range+0xbb/0x460 [btrfs]
[  352.992839]  btrfs_prealloc_file_range+0x10/0x20 [btrfs]
[  352.993430]  prealloc_file_extent_cluster+0x113/0x200 [btrfs]
[  352.994032]  relocate_file_extent_cluster+0x8d/0x470 [btrfs]
[  352.994646]  ? __btrfs_end_transaction+0x1c1/0x2e0 [btrfs]
[  352.995270]  relocate_data_extent+0x78/0xc0 [btrfs]
[  352.995895]  relocate_block_group+0x495/0x6f0 [btrfs]
[  352.996449]  btrfs_relocate_block_group+0x18e/0x280 [btrfs]
[  352.997014]  btrfs_relocate_chunk+0x4a/0xf0 [btrfs]
[  352.997591]  __btrfs_balance+0x8b7/0xbd0 [btrfs]
[  352.998174]  btrfs_balance+0x2de/0x5c0 [btrfs]
[  352.998758]  ? btrfs_balance+0x5c0/0x5c0 [btrfs]
[  352.999355]  balance_kthread+0x56/0x80 [btrfs]
[  352.999951]  kthread+0x113/0x130
[  353.000551]  ? kthread_create_worker_on_cpu+0x50/0x50
[  353.001125]  ret_from_fork+0x35/0x40
[  353.001673] Code: 48 c7 83 dc 00 00 00 00 00 00 00 48 c7 83 e4 00 00 00 00 00 00 00 c6 83 ec 00 00 00 00 c6 83 ed 00 00 00 00 e9 1b ff ff ff 0f 0b <0f> 0b 0f 0b 0f 0b 0f 0b 66 66 2e 0f 1f 84 00 00 00 00 00 90 0f
[  353.002298] Modules linked in: cmac ipt_MASQUERADE nf_nat_masquerade_ipv4 nf_conntrack_netlink nfnetlink xfrm_user xfrm_algo iptable_nat nf_nat_ipv4 xt_addrtype nf_nat br_netfilter bridge stp llc af_packet nf_log_ipv6 xt_comment nf_log_ipv4 nf_log_common xt_LOG xt_limit vmnet(O) fuse vmw_vsock_vmci_transport vsock vmw_vmci wireguard(O) ip6_udp_tunnel udp_tunnel vmmon(O) ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT xt_pkttype xt_tcpudp iptable_filter ip6table_mangle nf_conntrack_netbios_ns nf_conntrack_broadcast nf_conntrack_ipv4 nf_defrag_ipv4 ip_tables xt_conntrack nf_conntrack libcrc32c ip6table_filter ip6_tables x_tables bnep nls_iso8859_1 nls_cp437 vfat fat squashfs loop btusb btrtl btbcm btintel bluetooth ecdh_generic snd_hda_codec_hdmi msr snd_hda_codec_realtek snd_hda_codec_generic
[  353.005111]  iTCO_wdt iTCO_vendor_support mei_wdt ppdev dell_wmi sparse_keymap wmi_bmof mxm_wmi dell_laptop ledtrig_audio dell_smbios dell_wmi_descriptor dcdbas dell_smm_hwmon arc4 snd_hda_intel intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel snd_hda_codec snd_hda_core kvm snd_hwdep iwlmvm irqbypass snd_pcm joydev mac80211 e1000e pcspkr rtsx_pci_ms i2c_i801 ptp memstick pps_core snd_timer iwlwifi mei_me mei thermal intel_pch_thermal cfg80211 snd int3403_thermal parport_pc parport int3402_thermal processor_thermal_device dell_smo8800 video int340x_thermal_zone soundcore wmi tpm_crb dell_rbtn intel_soc_dts_iosf int3400_thermal acpi_thermal_rel rfkill battery acpi_pad ac button dm_crypt algif_skcipher af_alg uas usb_storage mmc_block hid_logitech_hidpp hid_logitech_dj hid_generic
[  353.008487]  usbhid nvidia_drm(PO) nvidia_modeset(PO) btrfs xor zstd_decompress zstd_compress xxhash crc32_pclmul crc32c_intel raid6_pq ghash_clmulni_intel rtsx_pci_sdmmc pcbc mmc_core nvidia_uvm(PO) serio_raw nvidia(PO) aesni_intel aes_x86_64 crypto_simd rtsx_pci cryptd glue_helper ahci drm_kms_helper libahci xhci_pci xhci_hcd drm drm_panel_orientation_quirks syscopyarea sysfillrect sysimgblt fb_sys_fops ipmi_devintf usbcore ipmi_msghandler sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua efivarfs
[  353.011341] ---[ end trace 554c2941265662db ]---
[  353.012472] RIP: 0010:create_reloc_root+0x1dc/0x1f0 [btrfs]
[  353.013566] RSP: 0018:ffffc9000a7bf9d0 EFLAGS: 00010282
[  353.014556] RAX: 00000000ffffffef RBX: ffff880ff06daa00 RCX: ffffea0040a6795f
[  353.015556] RDX: 0000000000000013 RSI: ffff880dfa0d8f50 RDI: 0000000000000286
[  353.016770] RBP: ffff880faac771e0 R08: ffff880e0415cea0 R09: 0000000000000000
[  353.017814] R10: 0000000000000028 R11: ffff880dfa0d8f50 R12: fffffffffffffff7
[  353.018814] R13: ffff880feb532000 R14: ffff880fefbdb800 R15: 00000000000c4000
[  353.019821] FS:  0000000000000000(0000) GS:ffff881082500000(0000) knlGS:0000000000000000
[  353.020877] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  353.021907] CR2: 000055f50cacc470 CR3: 000000000200a004 CR4: 00000000003606e0
[  353.022890] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  353.023866] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

I've experienced this exact same error at relocation.c:1449 running "4.12.14-lp150.12.58-default #1 openSUSE Leap 15.0". The call to btrfs_insert_root() within create_reloc_root() returns an error code which causes the subsequent BUG_ON() assertion to fail.

I'm hopeful that the following commit will fix this issue.

https://lkml.org/lkml/2019/6/7/720
https://github.com/torvalds/linux/commit/30d40577e322b670551ad7e2faa9570b6e23eb2b

Also experienced the same problem on kernel 4.12.14-lp151.28.4-default, openSUSE Leap 15.1. 

These two requests should integrate the commit above and fix it:
https://build.opensuse.org/request/show/710395
https://build.opensuse.org/request/show/710403

The workaround for now, is to mount with "skip_balance" option.

Few moments ago, SLES 12.3 all updates installed.

[528112.786104] BUG: unable to handle kernel paging request at ffff880273fa9b40
[528112.786401] IP: btrfs_init_reloc_root+0x2b/0xa0 [btrfs]
[528112.786584] PGD 200c067 P4D 200c067 PUD 2fed1c067 PMD 2feb7c067 PTE 8010000273fa9065
[528112.786842] Oops: 0003 [#1] SMP NOPTI
[528112.786948] CPU: 4 PID: 574 Comm: systemd-journal Not tainted 4.12.14-95.24-default #1 SLE12-SP4
[528112.787181] task: ffff8802ed710bc0 task.stack: ffffc90042bc8000
[528112.787362] RIP: e030:btrfs_init_reloc_root+0x2b/0xa0 [btrfs]
[528112.787575] RSP: e02b:ffffc90042bcbbe8 EFLAGS: 00010286
[528112.787722] RAX: ffff880273fa9800 RBX: ffff8802ea8eb800 RCX: 0000000000000000
[528112.787917] RDX: 0000000000118121 RSI: ffff8802ea8eb800 RDI: ffff88027f81e2d0
[528112.788114] RBP: ffff88027f81e2d0 R08: 0000000000000001 R09: ffff88027f81e2d0
[528112.788355] R10: 00000000000c0000 R11: ffff8802ec1b20c8 R12: ffff8802e9ba0000
[528112.788626] R13: ffff8802e9ba0078 R14: 0000000000000000 R15: ffff8802ed710bc0
[528112.788878] FS:  00007f4611f4c840(0000) GS:ffff8802f0900000(0000) knlGS:0000000000000000
[528112.789138] CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[528112.789314] CR2: ffff880273fa9b40 CR3: 0000000185a42000 CR4: 0000000000040660
[528112.789604] Call Trace:
[528112.789698]  record_root_in_trans+0xa9/0xf0 [btrfs]
[528112.789869]  btrfs_record_root_in_trans+0x4a/0x70 [btrfs]
[528112.790073]  start_transaction+0xab/0x440 [btrfs]
[528112.790217]  btrfs_dirty_inode+0x49/0xe0 [btrfs]
[528112.790379]  file_update_time+0xa6/0xf0
[528112.790558]  btrfs_page_mkwrite+0x129/0x490 [btrfs]
[528112.790712]  do_page_mkwrite+0x31/0x70
[528112.790857]  do_wp_page+0x43f/0x570
[528112.790968]  __handle_mm_fault+0x793/0xef0
[528112.791085]  handle_mm_fault+0xc4/0x1d0
[528112.791197]  __do_page_fault+0x1f3/0x4c0
[528112.791310]  do_page_fault+0x2b/0x70
[528112.791445]  ? do_syscall_64+0x9a/0x160
[528112.791560]  ? page_fault+0x2f/0x50
[528112.791685]  page_fault+0x45/0x50
[528112.791782] RIP: 2ee0:0x7fffd27e03d8
[528112.791883] RSP: cd92ee0:0000000000000001 EFLAGS: 55ce0cd92ee0
[528112.791886] Code: 0f 1f 44 00 00 41 55 41 54 55 48 89 fd 53 48 8b 86 f0 01 00 00 48 89 f3 48 8b 88 a0 cb 00 00 48 8b 46 18 48 85 c0 74 13 48 8b 17 <48> 89 90 40 03 00 00 5b 5d 41 5c 31 c0 41 5d c3 48 85 c9 74 f2 
[528112.792632] Modules linked in: nfsv3 rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache af_packet iscsi_ibft iscsi_boot_sysfs xenfs xen_privcmd intel_rapl sb_edac x86_pkg_temp_thermal coretemp crc32_pclmul ghash_clmulni_intel pcbc xen_netfront aesni_intel aes_x86_64 crypto_simd glue_helper cryptd pcspkr nfsd auth_rpcgss nfs_acl lockd grace sunrpc btrfs xor raid6_pq xen_blkfront crc32c_intel sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua scsi_mod autofs4
[528112.793813] Supported: Yes
[528112.793895] CR2: ffff880273fa9b40
[528112.793992] ---[ end trace d4d5746ac351a11a ]---
[528112.794139] RIP: e030:btrfs_init_reloc_root+0x2b/0xa0 [btrfs]
[528112.794296] RSP: e02b:ffffc90042bcbbe8 EFLAGS: 00010286
[528112.794508] RAX: ffff880273fa9800 RBX: ffff8802ea8eb800 RCX: 0000000000000000
[528112.794722] RDX: 0000000000118121 RSI: ffff8802ea8eb800 RDI: ffff88027f81e2d0
[528112.794915] RBP: ffff88027f81e2d0 R08: 0000000000000001 R09: ffff88027f81e2d0
[528112.795108] R10: 00000000000c0000 R11: ffff8802ec1b20c8 R12: ffff8802e9ba0000
[528112.791886] Code: 0f 1f 44 00 00 41 55 41 54 55 48 89 fd 53 48 8b 86 f0 01 00 00 48 89 f3 48 8b 88 a0 cb 00 00 48 8b 46 18 48 85 c0 74 13 48 8b 17 <48> 89 90 40 03 00 00 5b 5d 41 5c 31 c0 41 5d c3 48 85 c9 74 f2 
[528112.792632] Modules linked in: nfsv3 rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache af_packet iscsi_ibft iscsi_boot_sysfs xenfs xen_privcmd intel_rapl sb_edac x86_pkg_temp_thermal coretemp crc32_pclmul ghash_clmulni_intel pcbc xen_netfront aesni_intel aes_x86_64 crypto_simd glue_helper cryptd pcspkr nfsd auth_rpcgss nfs_acl lockd grace sunrpc btrfs xor raid6_pq xen_blkfront crc32c_intel sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua scsi_mod autofs4
[528112.793813] Supported: Yes
[528112.793895] CR2: ffff880273fa9b40
[528112.793992] ---[ end trace d4d5746ac351a11a ]---
[528112.794139] RIP: e030:btrfs_init_reloc_root+0x2b/0xa0 [btrfs]
[528112.794296] RSP: e02b:ffffc90042bcbbe8 EFLAGS: 00010286
[528112.794508] RAX: ffff880273fa9800 RBX: ffff8802ea8eb800 RCX: 0000000000000000
[528112.794722] RDX: 0000000000118121 RSI: ffff8802ea8eb800 RDI: ffff88027f81e2d0
[528112.794915] RBP: ffff88027f81e2d0 R08: 0000000000000001 R09: ffff88027f81e2d0
[528112.795108] R10: 00000000000c0000 R11: ffff8802ec1b20c8 R12: ffff8802e9ba0000

I consider it as a very serious bug. My problem was indeed that the drive ran out of space.
The only one solution [but only for few weeks/months] would be I run every day the balance.

I added additional space to the unit, extended correctly the size and now since more than 4 weeks there is no problem.

Why it is a bug:
The system cannot crash with above error messages and stop working. It has to bring the message that the drive is out of space, or it needs a balance or what ever.

I am now changing all btrfs drives to ext4. btrfs is apparently not stable enough and a toy for kindergarten environments but not for servers which are used for a serious work. At least SUSE is not capable to fix this.

They refer only to their paid support but this is ridiculous: Having very serious bugs and then ask for money? Good work guys!

I see the same issues on Ubuntu 20.04 with kernel 5.4.0-42. also with a debian buster rescue system from which I booted for recovery. Unfortunately,  I didn't note down the kernel version. Here is the backtrace from dmesg:

[Sun Aug  9 12:21:35 2020] ------------[ cut here ]------------
[Sun Aug  9 12:21:35 2020] kernel BUG at fs/btrfs/relocation.c:2626!
[Sun Aug  9 12:21:35 2020] invalid opcode: 0000 [#1] SMP PTI
[Sun Aug  9 12:21:35 2020] CPU: 1 PID: 4537 Comm: btrfs-balance Tainted: G           O      5.4.47 #1
[Sun Aug  9 12:21:35 2020] Hardware name: FUJITSU D3401-H1/D3401-H1, BIOS V5.0.0.11 R1.14.0 for D3401-H1x                    06/09/2016
[Sun Aug  9 12:21:35 2020] RIP: 0010:select_reloc_root+0x5b/0x19f [btrfs]
[Sun Aug  9 12:21:35 2020] Code: c0 c7 44 24 04 00 00 00 00 e8 8b 9d 17 e1 48 89 df 4c 89 f6 48 8d 54 24 04 e8 9c e6 ff ff 48 8b 58 60 48 89 c5 48 85 db 75 02 <0f> 0b 48 8b 43 20 a8 02 75 02 0f 0b 48 83 bb df 01 00 00 f8 75 45
[Sun Aug  9 12:21:35 2020] RSP: 0018:ffff8887e0b0bb20 EFLAGS: 00010246
[Sun Aug  9 12:21:35 2020] RAX: ffff8887dfab5280 RBX: 0000000000000000 RCX: 0000000000000000
[Sun Aug  9 12:21:35 2020] RDX: ffff8887e0b0bb24 RSI: ffff8887e0b0bc10 RDI: ffff8887dfab52c0
[Sun Aug  9 12:21:35 2020] RBP: ffff8887dfab5280 R08: ffff8887dfab52c0 R09: ffffffffa0491e7e
[Sun Aug  9 12:21:35 2020] R10: ffff8887f4ba7e70 R11: ffff8888090ed158 R12: ffff8887dfab5280
[Sun Aug  9 12:21:35 2020] R13: ffff8887fd330800 R14: ffff8887e0b0bc10 R15: ffff8887e7fa66e8
[Sun Aug  9 12:21:35 2020] FS:  0000000000000000(0000) GS:ffff88880e240000(0000) knlGS:0000000000000000
[Sun Aug  9 12:21:35 2020] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[Sun Aug  9 12:21:35 2020] CR2: 000055b4d5b7cfe0 CR3: 000000000200a004 CR4: 00000000003606e0
[Sun Aug  9 12:21:35 2020] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[Sun Aug  9 12:21:35 2020] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[Sun Aug  9 12:21:35 2020] Call Trace:
[Sun Aug  9 12:21:35 2020]  do_relocation+0xb6/0x4c2 [btrfs]
[Sun Aug  9 12:21:35 2020]  ? calcu_metadata_size.isra.36.constprop.42+0x9e/0xc4 [btrfs]
[Sun Aug  9 12:21:35 2020]  ? do_raw_spin_lock+0x2f/0x5a
[Sun Aug  9 12:21:35 2020]  ? btrfs_block_rsv_refill+0x4b/0x8b [btrfs]
[Sun Aug  9 12:21:35 2020]  relocate_tree_blocks+0x301/0x427 [btrfs]
[Sun Aug  9 12:21:35 2020]  ? tree_insert+0x49/0x4e [btrfs]
[Sun Aug  9 12:21:35 2020]  ? add_tree_block.isra.38+0x11e/0x144 [btrfs]
[Sun Aug  9 12:21:35 2020]  relocate_block_group+0x279/0x49e [btrfs]
[Sun Aug  9 12:21:35 2020]  btrfs_relocate_block_group+0x15e/0x23d [btrfs]
[Sun Aug  9 12:21:35 2020]  btrfs_relocate_chunk+0x25/0x8c [btrfs]
[Sun Aug  9 12:21:35 2020]  btrfs_balance+0xaf0/0xd45 [btrfs]
[Sun Aug  9 12:21:35 2020]  ? btrfs_balance+0xd45/0xd45 [btrfs]
[Sun Aug  9 12:21:35 2020]  balance_kthread+0x32/0x46 [btrfs]
[Sun Aug  9 12:21:35 2020]  kthread+0xf5/0xfa
[Sun Aug  9 12:21:35 2020]  ? kthread_associate_blkcg+0x86/0x86
[Sun Aug  9 12:21:35 2020]  ret_from_fork+0x3a/0x50
[Sun Aug  9 12:21:35 2020] Modules linked in: btrfs xor zstd_decompress zstd_compress lzo_compress lzo_decompress zlib_deflate raid6_pq libcrc32c sd_mod ipmi_devintf ipmi_msghandler sg x86_pkg_temp_thermal intel_powerclamp kvm_intel kvm irqbypass crc32_pclmul crc32c_intel iTCO_wdt ghash_clmulni_intel aesni_intel crypto_simd psmouse ahci cryptd libahci i2c_i801 serio_raw glue_helper intel_pch_thermal evdev video thermal acpi_pad button fan jc42 ftsteutates nct6775 hwmon_vid coretemp ip_tables x_tables autofs4 e1000e
[Sun Aug  9 12:21:36 2020] ---[ end trace 442b443de6cecc6e ]---
[Sun Aug  9 12:21:36 2020] RIP: 0010:select_reloc_root+0x5b/0x19f [btrfs]
[Sun Aug  9 12:21:36 2020] Code: c0 c7 44 24 04 00 00 00 00 e8 8b 9d 17 e1 48 89 df 4c 89 f6 48 8d 54 24 04 e8 9c e6 ff ff 48 8b 58 60 48 89 c5 48 85 db 75 02 <0f> 0b 48 8b 43 20 a8 02 75 02 0f 0b 48 83 bb df 01 00 00 f8 75 45

as for the original reporter, only the mount option skip_balance makes the file system usable

Here is my bug report in the Ubuntu launchpad