dropdown menu pages do not load automatically

After updating tot WP 4.7.3 and updating plugins I encountered an error 500. I looked for the plugins taht were updated but did not support 4.7.3. en removed these and the site is up and running again. Unfortunately in the dropdown menu the sub pages do not load automatically anymore (you need to refresh the page each time you click a submenu page in order to open that specific page. Any idea what causes this problem and how to fix it?