@source https://wordpress.org/support/topic/wp-admin-doesnt-work-inside-frames-anymore
- open wp-includes/default-filters.php
- find "frame"
- you'll get two lines:
add_action( 'login_init', 'send_frame_options_header', 10, 0 );
add_action( 'admin_init', 'send_frame_options_header', 10, 0 );
- comment those lines out, upload the file again and it will work, after the cache has been refreshed.