OTRS custom crontab was lost

Q: what can I do to restore lost custom crontab of otrs user after update?
A: nothing

Sounds horrible, isn't it? You can only avoid it in the future.

Don't use anymore crontab -e command. Just create new file (e.g. zzz_mycustom_tasks) in the directory
/opt/otrs/var/cron/

and save all your custom tasks in the crontab style
# unlock every hour old locked tickets
35 * * * *  $HOME/bin/otrs.UnlockTickets.pl --timeout >> /dev/null

and run then 

# /opt/otrs/bin/Cron.sh restart

Enjoy!