javascript ajax generates error Client/server version mismatch: Unsupported client

I have a problem connecting javascript and c# (Windows forms) using http requests. I have a JS code like this:

ext.get_led = function ( callback) {
        // Make an AJAX call to the Open Weather Maps API
        $.ajax({
            url: 'localhost:12754/poll',
            dataType: 'text',
            success: function (data) {
                // Got the data - parse it and return the temperature
                callback(data);
            },
            failure: function () {
                callback("Null");
            }
        });
    };

that is supposed to send an ajax request to the Windows Forms app. The Problem is that whenever I try to execute this function, the Javascript console shows an error:

In the link it gave me (http://ift.tt/1eceNme) there's this: "Client/server version mismatch: Unsupported client"

My c# code looks like that:

public class MyHttpServer : HttpServer
        {
            public int busy;
            public bool isSpeaking = false;
            public float result;
            public MyHttpServer(int port)
                : base(port)
            {
            }

            public override void handleGETRequest(HttpProcessor p)
            {
                if (p.http_url.Equals("/poll"))
                {
                    p.writeSuccess();
                    p.outputStream.WriteLine("SCRATCH");
                    p.httpHeaders = new Hashtable { { "Access-Control-Allow-Origin", "*" }, { "Content-Type", "text/plain"} };
                    return;
                }
            }
            public override void handlePOSTRequest(HttpProcessor p, StreamReader inputData)
            {

            }
        }

There are another 2 classes in the c# code, but their code is very long, so I won't put it here. I once tested the whole system with communication between 2 apps (not app-web), and it was working perfectly. But, when I wanted my app to communicate with the web, it refuses. The error says something is wrong with my client, but I have no idea what (Also the client - ajax - was tested and working fine).

Whoever helps, thank you.