Warning - Invalid account: 'root' not allowed to execute cronjobs

Dears,

I got "Warning - Invalid account: 'root' not allowed to execute cronjobs" warning message after executing the "crontab -e" command by root user. This actually prevent root cron jobs from running. However, with the "admin" user, crontab command works fine.

Hint: the root user is not listed on "/etc/cron.d/cron.deny" file. Moreover, the root account is not locked on "/etc/shadow" file. Also, I have tried to create "/etc/cron.d/cron.allow" file and restart cron service but the same issue still appear.

I have compared executing this command in another server by root user (with same OS version). It works fine on the other server.