I randomly decided to check my interbase_log file. So I 'tail -f'd it.
And this is what popped up once every second:

warthog (Server) Wed Apr 12 09:39:22 2000
SERVER/process_packet: connection rejected for interbase

And occasionally this one too:

warthog (Client) We Apr 12 09:46:46 2000
/usr/interbase/bin/ibguard: guardian starting bin/ibserver

I just installed it on this machine (I had to give up the old machine I
had it installed on) and can't ibmgr to start the server. The 'interbase'
user owns the whole interbase directory, what did I forget to do? I had it
working fine on my old machine.