My hoster updated the Linux kernel on my Debian system to 2.6.18-028stab070.5. After the update it wasn't possible to connect by ssh anymore.
Booted the system in recovery mode. /var/log/auth.log showed the following errors:
error writing /proc/self/oom_adj: Operation not permitted error: Bind to port 22 on a.b.c.d failed: Cannot assign requested address. fatal: Cannot bind any address.
The following changes fixed the problem: