Form submit button with ajax, replace form with another one (error 500)
You need to add your namespaced class to the callback property.
For example:

'callback' =&gt; 'Drupal\mymodule\Form\ProjektForm::form_ajax_submit'


or  

'callback' =&gt; [$this, 'form_ajax_submit']


You also need to make sure the wrapper and the prefix html id are the same.

Finally, after you do all that, do check your logs for more details on the error that caused the 500 response.

Good luck!