CP: AJAX Erros in Backend

When I do any change (assign a Instructor or change content) and click on Update, I get this error in the console (Google Chrome).

app.js:25 Uncaught TypeError: Cannot read property 'value' of null
https://waketo.de/wp-admin/admin-ajax.php Failed to load resource: the server responded with a status of 500 ()

In Firefox I get this,
POST https://waketo.de/wp-admin/admin-ajax.php

500 Internal Server Error
1,38s