[otrs] Use of uninitialized value

Hi folks! I upgraded to OTRS 3.2.3 from 3.1.6 two hours ago and have a problem. When writing a test email to my helpdesk, I get the following error emails from OTRS to our management email address. Subject:

Cron $HOME/bin/otrs.PostMasterMailbox.pl >> /dev/null

In the body, I have multiple lines saying:

Use of uninitialized value in hash element at /opt/otrs-3.2.3/Kernel/System/Ticket/Event/NotificationEvent.pm line 53.

If I take a look in my Apache logs, I find corresponding entries which look like:

[Wed Mar 20 16:11:35 2013] [error] [client 192.168.AAA.BBB] [Wed Mar 20 16:11:35 2013] index.pl: Use of uninitialized value in hash element at /opt/otrs-3.2.3/bin/cgi-bin/../../Kern el/System/Ticket/Event/NotificationEvent.pm line 53., referer: https://XXX.YYY.local/otrs/index.pl?Action=AgentTicketClose;TicketID=8281

The whole system seems to behave normally. It sends out emails, even the automatic replies for new incoming requests. But, for whatever reason, it produces these error emails. Has somebody an idea how to track down the problem to get rid of it? Regards Karsten