New users can't access admin

I’m having a problem where new users (Editors, Collaborator, Authors, etc.) cannot access the admin section, it only shows a blank page. This problem is not being experienced by the default ‘admin’ user.

I have been able to make it accessible to every users by removing all code relating to custom post types from the functions.php file. Either removing or commenting the code will make the admin area accessible to all.

I am trying to make sense of all of this. Any idea?