Hi Vince, thanks for reporting this, however, are you sure that the hide backend feature is breaking the functionality? I have seen this behavior and it was caused the Filter Long URLs.
Also, in this help document: https://mainwp.com/help/docs/am-i-allowed-to-change-wp-admin-and-wp-login-php-url/ we have a warning that changing the backend (wp-admin) slug may cause problems, so I would skip doing this. If you need additional security on your MainWP Dashboard, you can use the Clean and Lock extension to lock it down.