When the Menu Item is a sub-Item the URL for the Ajax call has became example.com/parent_menu_item/index.php which results in a 404. I have temporarily fixed it by moving the Menu Item up to the top level.
PS. This element still have severe performance issues, as it fetches all the rows in the database before the search term is entered by the user.
PS. This element still have severe performance issues, as it fetches all the rows in the database before the search term is entered by the user.