无法访问网络服务？问题是什么

public static string web Request(string AuthoUserName, string AuthoPassword, string sUserID, string sUserName, string sPlateNo, string sPlateCat, string sPlateCode, string sEmirate, string sCountry, string sCity, string sChessisNo, string sFetchURL)
{
    string str = string.Empty;
    try
    {
        Uri requestUri = new Uri(sFetchURL);
        if (!(requestUri.Scheme == Uri.UriSchemeHttp))
        {
            return str;
        }
        HttpWebRequest request = null;
        request = (HttpWebRequest) WebRequest.Create(requestUri);
        request.Method = "GET";
        request.ContentType = "text/xml;charset=\"utf-8\"";
        string str2 = "<soapenv:envelope xmlns:soapenv="\"http://schemas.xmlsoap.org/soap/envelope/\"" xmlns:ae="\"http://ae.gov.trf.inq.ws.TrafficInquiryService\""><soapenv:header><ae:password>" + AuthoPassword + "</ae:password><ae:username>" + AuthoUserName + "</ae:username></soapenv:header><soapenv:body><ae:getvehicledetails><getvehicledetailsrequest><userid>" + sUserID + "</userid><username>" + sUserName + "</username><plate><plateno>" + sPlateNo + "</plateno><platecategory>" + sPlateCat + "</platecategory><platecode>" + sPlateCode + "</platecode><platesource><emirate>" + sEmirate + "</emirate><country>" + sCountry + "</country><city>" + sCity + "</city></platesource></plate><chassisno>" + sChessisNo + "</chassisno></getvehicledetailsrequest></ae:getvehicledetails></soapenv:body></soapenv:envelope>";
        request.Method = "POST";
        request.ContentType = "application/soap+xml; charset=utf-8";
        request.Headers.Add("SOAPAction:\"\"");
        request.ContentLength = str2.Length;
        StreamWriter writer = new StreamWriter(request.GetRequestStream());
        writer.Write(str2);
        writer.Close();
        StreamReader reader = new StreamReader(request.GetResponse().GetResponseStream());
        while (!reader.EndOfStream)
        {
            str = str + reader.ReadLine();
        }
    }
    catch (WebException exception)
    {
        HttpWebResponse response = null;
        response = (HttpWebResponse) exception.Response;
        StreamReader reader2 = new StreamReader(response.GetResponseStream());
        while (!reader2.EndOfStream)
        {
            str = str + reader2.ReadLine();
        }
    }
    return str;
}

返回远程服务器返回错误（500）内部服务器错误此错误

it's return "the remote server returned an error (500) internal server error" This error