why NotFound error occur in REST services with windows Phone app?

i tried to connect REST web servie from windows phone 8 application.

it was working proberly for weeks but after no change in it I get this generic error :

System.Net.WebException: The remote server returned an error: NotFound.

i tried to test it by online REST Clients and services works properly

i tried to handle Exception and parse it as webException by this code :

    var we = ex.InnerException as WebException;
    if (we != null)
    {
        var resp = we.Response as HttpWebResponse;
        response.StatusCode = resp.StatusCode;

and i get no more information and final response code is : "NotFound"

any one have any idea about what may cause this error?

there is already a trusted Certificate implemented on the server . the one who has the server suggested to have a DNS entry for the server, this entry should be at the customer DNS or in the phone hosts file .that what i done and worked for awhile but now it doesn't work however i checked that there is no thing changed

this is sample for Get Request it works proberly on Windwos Store apps :

async Task<object> GetHttps(string uri, string parRequest, Type returnType, params string[] parameters)
        {
            try
            {
                string strRequest = ConstructRequest(parRequest, parameters);
                string encodedRequest = HttpUtility.UrlEncode(strRequest);

                string requestURL = BackEndURL + uri + encodedRequest;

                HttpWebRequest request = HttpWebRequest.Create(new Uri(requestURL, UriKind.Absolute)) as HttpWebRequest;

                request.Headers["applicationName"] = AppName;
                request.Headers["applicationPassword"] = AppPassword;
                if (AppVersion > 1)
                    request.Headers["applicationVersion"] = AppVersion.ToString();
                request.Method = "GET";

                request.CookieContainer = cookieContainer;

                var factory = new TaskFactory();
                var getResponseTask = factory.FromAsync<WebResponse>(request.BeginGetResponse, request.EndGetResponse, null);
                HttpWebResponse response = await getResponseTask as HttpWebResponse;
              //  string s = response.GetResponseStream().ToString();
                if (response.StatusCode == HttpStatusCode.OK)
                {
                    XmlSerializer serializer = new XmlSerializer(returnType);
                    object obj = serializer.Deserialize(response.GetResponseStream());
                    return obj;
                }
                else
                {
                    var Instance = Activator.CreateInstance(returnType);
                    (Instance as ResponseBase).NetworkError = true;
                    (Instance as ResponseBase).StatusCode = response.StatusCode;
                    return Instance;
                }

            }
            catch (Exception ex)
            {
                return HandleException(ex, returnType);
            }
        }

i tried to monitor connections from Emulator and i found this error in connection :

**

Authentication failed because the remote party has closed the transport stream.

**