CodeLover

I tried to trace the root of the problem.

I found that this problem arises because the module Advanced Access Manager (aam) overriding wp_die function:

// wp die hook
add_filter ('wp_die_handler', array ($ this, 'wpDie'), 10);

As a result, any errors that occur on the backend will be handled by the wpDie function
(example: upload a file that is not an image as a background on the menu appreance: Background):

     public function wpDie ($ function) {
         $ redirect = aam_Core_ConfigPress :: getParam ('backend.access.deny.redirect');
         $ message = aam_Core_ConfigPress :: getParam (
                         'backend.access.deny.message', __ ('Access Denied', 'aam')
         );
        
         if (filter_var ($ redirect, FILTER_VALIDATE_URL)) {
             wp_redirect ($ redirect);
             exit;
         } Elseif (is_int ($ redirect)) {
             wp_redirect (get_post_permalink ($ redirect));
             exit;
         } Else {
             call_user_func ($ function, $ message, '', array ());
         }
     }

As you can can see, this function will generate an error ‘Access Denied’

For temporary solution, I suggest making this line as a comment:
add_filter (‘wp_die_handler’, array ($ this, ‘wpDie’), 10);

I will try to ask the purpose of the filter wp die hook aam forum.