If you get 406 Not Acceptable I fixed it doing this.

var headers = form.headers;

Dictionary<string, string> ht = new Dictionary<string, string>();
ht["Accept"] = "*/*";
ht["Accept-Encoding"] = "gzip, deflate";
ht["User-Agent"] = "runscope/0.1";
ht["Content-Type"] = form.headers["Content-Type"].Replace("\"","");

WWW www = new WWW(url, form.data, ht);

This is also a workaround to Generic/unknown HTTP error using UnityWebRequest.

Still happening in 2017.3.1f1

The first thing to point out is that UnityWebRequest actually returns the response in

request.downloadHandler.text

While only returning the general error information in

request.error
_____________________________________

Now if your case is like mine and you upgraded Unity only to have all your networking code break, the error you'll likely see is related to Content-Length

This is because for both the WWW and UnityWebRequest class now have chunkedTransfer on by default. This appears to screw up how content length is sent over and causes the error.

While chunkedTransfer can currently be disabled for UnityWebRequest, it cannot be disabled for the WWW class at the time of this writing. For me this meant rewriting all my old network code to use UnityWebRequest instead and set

myUnityWebRequest.chunkedTransfer = false;

Note: UnityWebRequest does not currently support POSTs with no data. You still need to use the WWW class for this.

Still get this in 2017.3.0f3

I still get this in 2017.3.1f1

This basically halts production for server based development for everyone concerened.

Could not use WWW
Could not use UnityWebRequest
Both for POST and PUT

Tried changing content type header.... nothing.

This isn't fixed. Still getting it in 2017.3.0f3.

i get same issue in 2017.3.0f3

i still get this in 2017.3.0p3

I also still get this on 2017.3.0p2

same for me here with Unity 2017.3 p2