Hi Aljaz,
I am not sure if that works but you can try to apply the recommendations in this thread:
https://forum.mendix.com/link/questions/95290.
If I correctly remember especially this comment on that thread (“HTTP Headers are correct. Instead of using a CONSTANT to deliver the URL, I put the URL in the Location field of the Call REST directly. Now the call is working well! I wonder why that is. Anyway, thanks for your comments!”) helped me to solve the error.
Maybe that helps you too.