I'm not a fan of the other suggestions to manipulate crontab directly because (in Linux):
otrs/bin/Cron.sh restart otrs
will concat all entries [that don't have .dist] on otrs/var/cron to the otrs user's cron (to run in the otrs context, obeying otrs' HOME path).
to verify otrs' cron:
sudo crontab -l -u otrs | more
The error message is a symptom (Can't create a new process because the other process is still running). It is what it says.
More to the point, the PostmasterMailbox.pl *is* running too often. This is likely either because it's running as root AND otrs or because a single instance is hung.
check /var/spool/cron/crontabs/* for multiple instances of PostmasterMailbox.pl (but use crontab -e -u username to edit these!)
also, check /etc/crontab and /etc/cron* for possible manual entries and remove them, especially if you've followed other people's instructions.
Check ps auxwww | grep PostmasterMailbox for running processes.