Web服务:403禁止的错误

我正试图将服务中的一些数据访问到我的windows service中。这是其中一个线程中的代码：

HttpWebRequest webrequest = HttpWebRequest.Create(url + "/data.json?auth_token=" + authenticationToken);
ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;
System.Net.ServicePointManager.ServerCertificateValidationCallback =
               ((sender, certificate, chain, sslPolicyErrors) => true);
webrequest.Method = "GET";
webrequest.ContentType = "application/jsonp";
HttpWebResponse webresponse = (HttpWebResponse) webrequest.GetResponse();
Encoding enc = System.Text.Encoding.GetEncoding("utf-8");
StreamReader loResponseStream = new StreamReader(webresponse.GetResponseStream(), enc);
result = loResponseStream.ReadToEnd();

它以前工作正常。但几天以来，它给出了错误：The remote server returned an error: (403) Forbidden. at System.Net.HttpWebRequest.GetResponse()

编辑：

如果我通过浏览器尝试请求，我会得到正确的响应，但不会通过windowa服务！