远程服务器返回一个错误:(500)内部服务器错误

为什么我得到"远程服务器返回一个错误:(500)内部服务器错误。"这个错误是我的代码

    public override object Request()
    {
        Response = "";
        RequestTime = DateTime.Now;
        var url = _service.Url;
        HttpWebResponse responseObj = null;
        try
        {
            Uri requestUri = null;
            Uri.TryCreate(url, UriKind.Absolute, out requestUri);

            CredentialCache cc = new CredentialCache();
            NetworkCredential networkCredential = new NetworkCredential("username",
                                         "password", url);
            Uri uri = new Uri(url);
            cc.Add(uri, "Basic", networkCredential);
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(uri);
            request.Proxy = new WebProxy(url);
            request.Credentials = cc;
            request.Method = WebRequestMethods.Http.Post;

            Response = request.GetResponse();

        }
        catch (Exception x)
        {
            Logger.append(x.Message, Logger.ERROR);
        }
        Duration = DateTime.Now.Subtract(RequestTime).TotalMilliseconds;
        return Response;
   }

我尝试直接从浏览器输入url，它可以工作。