CentOS 5.4 - firebird-2.1.3.18185.0-2.el5 classic

I recently had a problem on this machine and the database was no longer
working. There is a daemon that is regularly connecting, updating and
disconnecting from the database, and it also launches nbackup hourly. I found
that the nbackup .delta file and the database both had timestamps of several
days ago. firebird.log contained the following error with the same timestamp
as the database and .delta file:

ISC_kill: process 20185 couldn't deliver signal 16 to process 12040.
permission denied

I stopped xinetd, killed all firebird processes, restarted xinetd, ran nbackup
with the -N option to unlock the database. Having validated the database with
gfix, I restarted the application and it's working again.

Can someone please explain what the ISC_kill error is all about, and what I
can do to prevent it from happening again?

Tom Munro Glass