User not allowed to use crontab (Folder Auto-Clearing)

Our users are receiving the following error when trying to save folder auto-clearing:

You (username) are not allowed to use this program (crontab)
See crontab(1) for more information
7 * * * * /home/username/.usermin/mailbox/


To fix this, login to Virtualmin and go to Webmin -> System -> Scheduled Cron Jobs, click "Control user access to cron jobs" and make sure all users are allowed.