Json body not supported?

I have tried to run some requests with Postman and including a body seems to be problematic:

• Requests towards https://demo.httprelay.io/proxy/zg5MD/time-remaining?until=1685909015

  • Works fine without a body
  • If a body is included ( a plain "{}"):
    • Even requests return : 400 Bad Request
    • Odd request work but content-length is 0
      • 

• Request towards a local proxy running with docker, server side is a request with from C#/UnityWebRequest (Unity game engine).

  • content-length is fine, but the connection closes before the content is downloaded (not sure if it closes early because of the client or the proxy)

Any clues on the reason and how it could be avoided? .