I have an issue when I logged in to Wp-admin dashboard page I can see the wordpress-admin bar on the top of the page but the dashboard page actually redirected to 404 error page. I have disabled all the plugins and activated theme to find the problem, and change the code inside .htaccess file to default wordpress code, but nothing worked. Also, I have tested to create the test.php page inside the wp-admin directory but it also returned to 404 page when I access to https://example.com/wp-admin/test.php. So I think the PHP is not working inside the wp-admin? Has anyone experienced this before and have any more ideas?