1)
You receive error [an error occurred while processing this directive] on main cpanel page. Mostly you receive this error because of main domains entry missing in cpanel users file.

If main domain name= domainname.com Please check following file
Vi /var/cpanel/users/username
Search line as per below
DNS=domainname.com
If above entry is not there add it in file.
If the entry is as per below
XDNS1=domainname.com
then properly changed it as per above.
It solves your issue.

If you are still facing problem please check ownership for the directory /home/username. Correct ownership is username.username.

2)
If client receives same error [an error occurred while processing this directive] when he try to add mail forwarders through cpanel then check ownership for the following file
/etc/valiases/domainname.com
Correct ownership is username.mail and correct file permission is 644.

It solves error and client is able to add forwarder through cpanel.