I just found something interessting. The following cronjob doesn't work:
cat ebox-mail
# Cron file for package zentyal-mail
# see if fetchmail daemon needs relaod each 10 minutes
1,11,21,31,41,51 * * * * root [ -x /usr/share/zentyal-mail/fetchmail-update ] && /usr/share/zentyal-mail/fetchmail-update
When excecuted as root: sudo [ -x /usr/share/zentyal-mail/fetchmail-update ] && /usr/share/zentyal-mail/fetchmail-update
I get: Cannot change user to ebox. Are you root? at /usr/share/perl5/EBox.pm line 178.
Could it indeed be a users problem?