Custom Action authentication debug

I want to send a Custom Action POST request to CRM during a journey workflow:  

1. jo_status_code for this sendout shows 400 without any further details. I asked support for help 
2. Additionally test authentication shows that the test is successful. Did you come across that this feature might be bugged?

   

    

So the error might be with my request structure. How would you futher debug? :(