Fastest way to debug network stuff:
Networking, UnityWebRequest, WWW, Postman, curl, WebAPI, etc:
https://discussions.unity.com/t/831681/2
https://discussions.unity.com/t/837672/2
And setting up a proxy can be very helpful too, in order to compare traffic:
https://support.unity.com/hc/en-us/articles/115002917683-Using-Charles-Proxy-with-Unity