We are using a RAID 1 with four 2TB hard drive on a Debian 7.1.
After about one year of working, there was an error in messages log and the filesystem was mounted read-only.

After that I restarted the system but that doesn't fix the bug. The btrfs-filesystem couldn't be mounted.

I attach four logs:

-> when the error occured
-> after the error occured
-> when rebooting the system
-> when I tried ti mount manually

Before I rebuilt the filesystem (formatting everything and creating a new btrfs-pool) I made a btrfs-image. You can download it at: 

https://hiperscan-download.de/software/public/btrfs_image

messages when the error occured:
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.567383] parent transid verify failed on 1750188515328 wanted 475562 found 475559
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614152] parent transid verify failed on 1750188515328 wanted 475562 found 475559
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614165] ------------[ cut here ]------------
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614186] WARNING: at fs/btrfs/super.c:255 __btrfs_abort_transaction+0x4c/0xcf [btrfs]()
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614187] Hardware name: System Product Name
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614190] Modules linked in: vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) xts gf128mul tun pc
i_stub cpufreq_powersave cpufreq_userspace cpufreq_stats cpufreq_conservative binfmt_misc fuse nfsd auth_rpcgss nfs_acl nfs lockd fscache su
nrpc ext2 dm_crypt loop firewire_sbp2 snd_hda_codec_hdmi radeon snd_hda_intel coretemp snd_hda_codec kvm_intel kvm ttm drm_kms_helper drm i7
core_edac ehci_pci i2c_algo_bit snd_hwdep iTCO_wdt iTCO_vendor_support lpc_ich mfd_core snd_pcm edac_core asus_atk0110 acpi_cpufreq evdev mp
erf i2c_i801 psmouse i2c_core pcspkr mxm_wmi serio_raw processor snd_page_alloc snd_timer button wmi thermal_sys snd soundcore microcode ext
4 crc16 jbd2 mbcache btrfs xor zlib_deflate raid6_pq libcrc32c dm_mod raid1 md_mod sd_mod crc_t10dif sg hid_generic usbhid hid firewire_ohci
 crc32c_intel firewire_core crc_itu_t sata_sil r8169 mii ahci libahci libata uhci_hcd ehci_hcd scsi_mod xhci_hcd usbcore usb_common [last un
loaded: vboxdrv]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614245] Pid: 2235, comm: btrfs-transacti Tainted: G           O 3.9.6 #1
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614247] Call Trace:
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614253] [<ffffffff8103d510>] ? warn_slowpath_common+0x76/0x8c
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614257] [<ffffffff8103d5be>] ? warn_slowpath_fmt+0x45/0x4a
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614268] [<ffffffffa01a8777>] ? __btrfs_abort_transaction+0x4c/0xcf [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614281] [<ffffffffa01bb32e>] ? btrfs_run_delayed_refs+0x237/0x394 [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614296] [<ffffffffa01c82ff>] ? btrfs_commit_transaction+0xaf/0x87a [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614310] [<ffffffffa01c7365>] ? join_transaction.isra.29+0x314/0x31e [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614314] [<ffffffff8105866b>] ? abort_exclusive_wait+0x79/0x79
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614329] [<ffffffffa01c92cc>] ? start_transaction+0x325/0x44d [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614332] [<ffffffff81049415>] ? ftrace_raw_event_timer_class+0x8e/0x8e
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614336] [<ffffffff8106144c>] ? should_resched+0x5/0x23
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614349] [<ffffffffa01c1d95>] ? transaction_kthread+0xe0/0x183 [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614363] [<ffffffffa01c1cb5>] ? try_to_freeze+0x31/0x31 [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614377] [<ffffffffa01c1cb5>] ? try_to_freeze+0x31/0x31 [btrfs]
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614380] [<ffffffff81057da4>] ? kthread+0x81/0x89
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614383] [<ffffffff81057d23>] ? __kthread_parkme+0x5d/0x5d
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614386] [<ffffffff8138b23c>] ? ret_from_fork+0x7c/0xb0
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614389] [<ffffffff81057d23>] ? __kthread_parkme+0x5d/0x5d
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614391] ---[ end trace c45c78ec20af1ebf ]---
Jun 25 17:02:25 hsad-srv-03 kernel: [870100.614395] btrfs is forced readonly
---------------------------------------------------------------------------------------
messages after the error before reboot:
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061426] Pid: 62, comm: kswapd0 Tainted: G        W  O 3.9.6 #1
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061427] Call Trace:
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061428] [<ffffffff8103d510>] ? warn_slowpath_common+0x76/0x8c
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061439] [<ffffffffa01d51ab>] ? btrfs_destroy_inode+0x5f/0x252 [btrfs]
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061441] [<ffffffff81119e30>] ? dispose_list+0x27/0x31
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061443] [<ffffffff8111aefe>] ? prune_icache_sb+0x2ab/0x2c6
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061445] [<ffffffff81109ed4>] ? prune_super+0xc3/0x12d
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061447] [<ffffffff810d0214>] ? shrink_slab+0x19b/0x287
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061449] [<ffffffff810d2a90>] ? kswapd+0x512/0x6fd
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061452] [<ffffffff810d257e>] ? try_to_free_pages+0x127/0x127
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061454] [<ffffffff810d257e>] ? try_to_free_pages+0x127/0x127
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061455] [<ffffffff81057da4>] ? kthread+0x81/0x89
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061458] [<ffffffff81057d23>] ? __kthread_parkme+0x5d/0x5d
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061459] [<ffffffff8138b23c>] ? ret_from_fork+0x7c/0xb0
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061461] [<ffffffff81057d23>] ? __kthread_parkme+0x5d/0x5d
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061463] ---[ end trace c45c78ec20af24c9 ]---
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061463] ------------[ cut here ]------------
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061473] WARNING: at fs/btrfs/inode.c:7924 btrfs_destroy_inode+0x76/0x252 [btrfs]()
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061474] Hardware name: System Product Name
Jun 25 23:02:13 hsad-srv-03 kernel: [891678.061475] Modules linked in: vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) xts gf128mul tun pci_stub cpufreq_powersave cpufreq_userspace cpufreq_stats cpufreq_conservative binfmt_misc fuse nfsd auth_rpcgss nfs_acl nfs lockd fscache sunrpc ext2 dm_crypt loop firewire_sbp2 snd_hda_codec_hdmi radeon snd_hda_intel coretemp snd_hda_codec kvm_intel kvm ttm drm_kms_helper drm i7core_edac ehci_pci i2c_algo_bit snd_hwdep iTCO_wdt iTCO_vendor_support lpc_ich mfd_core snd_pcm edac_core asus_atk0110 acpi_cpufreq evdev mperf i2c_i801 psmouse i2c_core pcspkr mxm_wmi serio_raw processor snd_page_alloc snd_timer button wmi thermal_sys snd soundcore microcode ext4 crc16 jbd2 mbcache btrfs xor zlib_deflate raid6_pq libcrc32c dm_mod raid1 md_mod sd_mod crc_t10dif sg hid_generic usbhid hid firewire_ohci crc32c_intel firewire_core crc_itu_t sata_sil r8169 mii ahci libahci libata uhci_hcd ehci_hcd scsi_mod xhci_hcd usbcore usb_common [last unloaded: vboxdrv]
------------------------------------------------------------------------------------------------------------------------------
messages at reboot:
Jun 26 07:14:41 hsad-srv-03 kernel: [   13.735382] btrfs: use lzo compression
Jun 26 07:14:41 hsad-srv-03 kernel: [   13.735386] btrfs: allowing degraded mounts
Jun 26 07:14:41 hsad-srv-03 kernel: [   13.735389] btrfs: disk space caching is enabled
Jun 26 07:14:41 hsad-srv-03 kernel: [   34.700453] parent transid verify failed on 1750188515328 wanted 475562 found 475559
Jun 26 07:14:41 hsad-srv-03 kernel: [   35.095814] parent transid verify failed on 1750188515328 wanted 475562 found 475559
Jun 26 07:14:41 hsad-srv-03 kernel: [   35.124511] btrfs: open_ctree failed
Jun 26 07:14:41 hsad-srv-03 kernel: [   35.133282] device fsid 526356ed-9708-40c2-83a1-82f3dbc8b72f devid 2 transid 476583 /dev/sde
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when I want to mount the btrfs: mount: wrong fs type, bad option, bad superblock on /dev/sde,
      missing codepage or helper program, or other error
      In some cases useful info is found in syslog - try
      dmesg | tail  or so