Roejack, did you solve this issue? I've run into what I think is exactly the same issue or something very similar and I'm struggling to find the source of my troubles. My sketch was working fine, then suddenly with no changes to the sketch it stopped working. I'm getting this 406 error. if I add a \r\n to the end of the url i'm posting I get a longer return error from the server. If I take the post url and just paste it into a browser it returns successful every time. I'm wondering if this is an issue in my code or is it a server issue? I was thinking maybe the server is looking for something new that the library doesn't provide? I hope you can help me.

Here is the error

Not Acceptable

An appropriate representation of the requested resource /dgf/plc/com/ could not be found on this server.

Additionally, a 406 Not Acceptable error was encountered while trying to use an ErrorDocument to handle the request.