Using the "Active Easy Admin" button I can switch between the Easy Admin and Advanced Admin from any page.
However, in the Easy Admin, if I am currently on the "My Post" tab or "New Page" tab and I click "Activate Advanced Theme", it takes me to a 404 page not found.
My Post tab -> Activate Advanced Theme = "/wp-admin/page-new.php?easyadmin=off" (404)
New Page tab -> Activate Advanced Theme = "/wp-admin/page-new.php?easyadmin=off" (404)
I don't believe wordpress has a page-new.php file. I believe the correct filename should be post-new.php