For whatever reason (I couldn't figure out in the last 2 entire days)
when using application/json the JSON.parse() function in the server
crashes (you also can't just take the requst body since it is still a
string). It works with plain text though so even though it's technically
incorrect to use text/plain we'll use it until somebody figures out what
the hell is wrong.
Do note that the same request (Content-Type also application/json) but
made from a REST client works perfectly fine.
Edit: Might be an issue in the frontend, see here for that.