KERNEL: assertion ((int)tcp_packets_in_flight(tp) >= 0) failed at net/ipv4/tcp_input.c (1274) Maybe it is somehow related to: http://www.mail-archive.com/netdev@vger.kernel.org/msg57851.html
Probably fixed by Ilpo in later kernel versions.
Another potential cause could be the problem reported in Bug #10384 The patch however, wouldn't work as is with 2.6.22.
Krzysztof, have you tested latest kernel by chance, has the problem gone away with all relevant patches incorporated?
Not yet, as I hit another problems so I have not been able to do it. Hopefully, there is only one left.
Fixed in more recent kernels.