To see the response clearly, put echo "Not Found"; before header("HTTP/1.0 404 Not Found"); in _kdkht-UFQ2L-BC0UR-HNGWS-8KUY8_.php

You will see the Not Found response in all direct call(GET & PUT) to _kdkht-UFQ2L-BC0UR-HNGWS-8KUY8_.php except the GET-&-the-correct-password case. In the GET-&-the-correct-password case, you will see a blank page and Console was cleared