WHM: Can't set default option to limit email

I can't set the option to limit outgoing mail in WHM.

 

Tweak Settings -> Restrict outgoing SMTP to root, exim, and mailman

 

Default is "on", but it's set to "off" (which I didn't set - it came like that with the CentOS 6 upgrade).
Set it to "on".
Message: There was an error updating “Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak)”:

No error given.

 

How can I fix this?