string strData = "";
try
{
    string RequestURL = ...;
    HttpWebRequest webRequest = ...;

    //Вот тут ошибка
    using (HttpWebResponse response = (HttpWebResponse)webRequest.GetResponse()) //ошибка
    {
        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
        {
            strData = reader.ReadToEnd();
            reader.Close(); reader.Dispose();
        }
        response.Close();
    }
}
catch { }
return strData;