I’ve noticed that on some new installs of cPanel 11.x + CentOS 5.0 there is a problem with cron jobs where the crons are not saved at all with no apparent reason and with no errors.
After some investigations I’ve found out that this is do to the fact that crontab doesn’t have the correct permissions.
It’s an easy fix and all you have to run as root is:
chmod a+s /usr/bin/crontab
Now try to add your crons again. It should work.