@imanuelr
You are getting 200 response code which means the requests are successful so it appears to be something deeper. You can inspect element in the browser, open the Network tab, run the AJAX request, click the request in the Network tab, click Response tab, and ensure the appropriate data is returned. If not, you will need to check your IIS error logs, PHP error logs, MySQL error logs, and console logs for any related errors. If you get no errors there then last thing you can do is some debugging in the code.
Cheers.
