Post a screenshot of your job. I am guessing you are leaving a field empty.
Aha, you are right. I didn't need a comment so left the field empty. Filling it with any character does the job right. It doesn't leave a notice that it needs to be filled. Thx for the plugin and pointing that out
Edit: the commentfield needs to be activated and filled with a character. Deleting the character (because you don't need it) still makes it possible to save the job. Without activation of the field it results in error 400.