I've now encountered the exact same problem with a user on SolidCP with MailEnable. As an admin I can add/change users no problem, but with their user account they're getting "You are not allowed to access this package": It's the exact same error message as Trevor posted with a screenshot earlier

I'm already using 1.4.4 so an upgrade isn't going to fix it.

This seems to be the same issue here - https://solidcp.com/forum/question/mailbox-issue/

Fix is apparently in 1.4.5beta, so I'll search how to obtain that.

Disabling "Disable Mailbox Size Edit" fixed the problem for now, so I'll wait for an official update.