Here's some more information on the setup:

The Firebird-version is LI-V1.5.0.4290 Firebird 1.5

It's running on quite an old Linux-server with Kernel version 2.4.20 (smp) on a
Red Hat Linux 9 (Shrike).
I'm planning to upgrade the server to an ubuntu or debian installation
soon, though...

There are no related messages in firebird.log, the last message is
three days old
and TCP/IP-related (receive in try_connect errno = 104).

The system is being used by approx. 10-17 users, with only a few databases
(one productive, a few for development) of small size (< 2GB).

If you need any other info, please let me know.

Thanks,

Daniel