I don’t know what it is, but this happened last Spring too. For reasons unknown, any and all SSH sessions I open to the firewall end up dying when I try to do anything interactive, like run top or use tunnels. Last time this happened, I tried everything I could think of, but nothing seemed to help. I finally rebooted and that resolved everything. At the time, I was using a much earlier version of OpenSSH and a different, 2.4.x series kernel. Today I’m using the current version of OpenSSH and 2.6.6. The only thing I can think of is either a kernel bug or an OpenSSH bug, or both.
I prefer not to reboot since nearly always there is a userspace resolvable issue somewhere that rebooting might only temporarily cover up, but in this case I found no other solution. I guess I’ll try and -rc kernel if it continues.
Actually, as it turns out, it was a PPPoE issue. Although for months it did not seem to matter, changing my MTU from 1500 to 1492 magically resolved this issue. I can’t imagine why it worked fine for six months and suddenly broken, but here it is, broken.