[PROBLEM]
Usually, the crontab for the system is located in "/etc/crontab" - but entries created with Webmin don't show up in there.
Where are they?
[SOLUTION]
- Open Webmin
- Open System/Scheduled Cron Jobs
- Click on "Module Config"
There you will find a setting called "Crontab Directory". Per default that's /var/spool/cron/crontabs
If you look in that folder, you will see subfolders per username that has cronjobs to run.