Вероятно были проблема с куками или сессией

CookieContainer cookies = new CookieContainer();
cookies.Add(new Cookie("ASP.NET_SessionId", sessionId));
var URL = "http://www.somesite.com";
HttpWebRequest webRequest = WebRequest.Create(URL) as HttpWebRequest;
webRequest.Credentials = new NetworkCredential(ExtAccountToUse, ExtPassword);
webRequest.CookieContainer = cookies; // note this
webRequest.Method = "POST";
webRequest.ContentType = "application/x-www-form-urlencoded";                
webRequest.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 6.0; sv-SE; rv:1.9.1b2) Gecko/20081201 Firefox/3.1b2";
webRequest.ContentLength = paramStream.Length;
byte[] paramStream = Encoding.ASCII.GetBytes("param1=value&param2=value2");
using (Stream stream = webRequest.GetRequestStream())
{
    stream.Write(paramStream, 0, paramStream.Length);
}

string result;
using (WebResponse response = request.GetResponse())
{
    using (var sr = new StreamReader(response.GetResponseStream()))
    {
        result = sr.ReadToEnd();
    }
}