I'm having trouble with my OTRS. Out of nowhere I'm getting Internal Server Error sometimes.

It happens when I click on the send botton.
I rebooted the server and it is now working again.
Do you know what log-files to look in to see what caused the error?
In the apache error log I find this:
[Thu Oct 22 12:47:27 2015] [error] panic: attempt to copy value HASH(0x7febcd8b9580) to a freed scalar 7febcdcabe48 at /usr/lib/perl5/DBI.pm line 1275.\n
[Thu Oct 22 12:47:41 2015] -e: Use of uninitialized value $Service{"Criticality"} in hash element at /opt/otrs//Kernel/Modules/AgentTicketActionCommon.pm line 1088.
[Thu Oct 22 12:47:41 2015] -e: Use of uninitialized value within @_ in list assignment at /opt/otrs//Kernel/System/DynamicField/Backend.pm line 411.
/Joakim