基础连接已经关闭：无法连接到远程服务器

try
{
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(serverURL + "/login.aspx");
ElinkLog.WriteEntry(request.RequestUri.ToString());
request.Headers.Set("Pragma","no-cache");
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
if(response.StatusCode!=HttpStatusCode.OK)
                                           ......
}
catch(WebException webException)
{
                                    //基础连接已关闭
ElinkLog.WriteEntry(webException.Status.ToString());
ElinkLog.WriteEntry(NetUtility.GetResponseString(webException.Response));
}