Did you end up finding the otrs log file? I recall that it should have been in /var/log but wasn't!? Alternatively it may be in the otrs user's $HOME/var/log I don't recall where otrs $HOME is? Was it /var/lib/otrs or something? Also the Apache log file may have something useful (/var/log/apache2/error.log)?

I did a quick google and it seems on face value like a somewhat common issue but with a range of causes none of which make any sense to me in the context of the TurnKey appliance (e.g. lots of what I read were issue which occur after upgrading an existing install).

Overall though; to me it suggests a bug in the Debian packaged version of OTRS or maybe an incompatibility between OTRS and one of it's dependencies (version mismatch or something like that). Sorry that I don't have a better answer...