远程服务器返回错误: (500) 内部服务器

在.net中发送 xml post请求和接受xml 的post请求时，遇到“远程服务器返回错误: (500) 内部服务器错误”。
我在post请求的方法加入"text/xml"  也不行 求大神指教
post的方法如下：
string GetResponse(string url, string param, string Method)
        {
            byte[] postData = Encoding.UTF8.GetBytes(param);            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            req.Method = "POST";
            req.KeepAlive = true;
            req.Timeout = 300000;
            req.ContentType = "application/x-www-form-urlencoded;charset=utf-8;text/xml";
            req.ContentLength = postData.Length;
            req.Headers.Add("api-version", "2.0");            Stream reqStream = req.GetRequestStream();            reqStream.Write(postData, 0, postData.Length);            reqStream.Close();
            HttpWebResponse rsp = (HttpWebResponse)req.GetResponse();            Stream stream = rsp.GetResponseStream();            StreamReader sr = new StreamReader(stream, Encoding.GetEncoding("utf-8"));
            string result = sr.ReadToEnd();
            sr.Close();
            stream.Close();            return result;
        }