OTRS needs a few system cronjobs to unlock tickets with lock timeouts, send reminder notifications and some other stuff.
There are several OTRS default cronjobs in $OTRS_HOME/var/cron/*.dist. Make copies of all of the default cronjobs:
cd var/cron for foo in *.dist; do cp $foo `basename $foo .dist`; done |
cd var/cron copy *.dist *. |
Note: Install this cronjobs as OTRS user.