Custom /wp-admin/ URL should not affect the MainWP functionality in any way.
There is probably something else causing this error. Please try to deactivate all security plugins on the child site temporarily and see if that helps.
Custom /wp-admin/ URL should not affect the MainWP functionality in any way.
There is probably something else causing this error. Please try to deactivate all security plugins on the child site temporarily and see if that helps.