My Windows Phone app Get empty response (404 Not Found) Scond time, work's great first time;And always work fine if without SSL

Please try to use this solution.

        public async Task<string> SendJSONData3(string urlToCall, string JSONData)    {        string UserName = "XXXXXXXXX";        string Password = "XXXXXXXXX";        var httpWebRequest = (HttpWebRequest)WebRequest.Create(urlToCall);        httpWebRequest.Credentials = new NetworkCredential(UserName, Password);        httpWebRequest.ContentType = "text/json";        httpWebRequest.Method = "POST";        using (var streamWriter = new StreamWriter(await httpWebRequest.GetRequestStreamAsync()))        {            string json = JSONData;            streamWriter.Write(json);            streamWriter.Flush();        }        var httpResponse = (HttpWebResponse)await httpWebRequest.GetResponseAsync();        using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))        {            var result = streamReader.ReadToEnd();            return result;        }    }

A couple of ideas:

• Do not use the .Result property. Just use await instead to avoid deadlocks.
• Remove the additional space in front of the media type parameter " application/json"
• Enable logging on the webserver and see if the second request arrives on the server.
• Get a network trace, for example with Wireshark or Fiddler.
• Try puting WebRequest.RegisterPrefix("https://", WebRequestCreator.ClientHttp); in your initialization code, as proposed in this answer.