ipnat ftp proxy suddenly stopped working

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I'm facing a weird problem where on one of my servers, all of a sudden, I'm 
unable to establish connections to the FTP port on another server (Network is 
unreachable). The problem goes away when I disable ipnat (which has a proxy 
rule for ftp) and re-appears if I re-enable ipnat.

Since the machine in question is my prototype monitoring server, I can say 
for sure the problem occured out of the blue, without any configuration 
change or whatever, last night around 03:25.

Any hints? Some kind of resource shortage?

As this is a development machine, I can drop to DDB or whatever to track down 
the issue.