Accessing the secured site using Webrequest

Try to close the request stream before retrieving the response.
You can accomplish this by using "using" blocks:

public string login(string url, string postData)
{
    byte[] requestData = Encoding.UTF8.GetBytes(postData);
    
    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
    webRequest.Method = "POST";
    webRequest.ContentType = "application/x-www-form-urlencoded";
    webRequest.ContentLength = requestData .Length;
 
    using (Stream requestStream = webRequest.GetRequestStream())
    {
        webpageStream.Write(requestData, 0, requestData.Length);
    } //this will close the requestStream automatically

    WebResponse webpageResponse = webRequest.GetResponse();
 
    using (responseReader = new StreamReader(webpageResponse.GetResponseStream()))
    {
        return webpageReader.ReadToEnd();
    } //the reader will close the underlaying stream too
}    

(Besides, I've done some style improvements you should adopt, the code will be a far more readable)

For more infos, see a Tutorial on MSDN.[^]

And:

Response.Write(responseFromServer);

Never do that!
What is Response? (A rhetorical question)
This is called bad side effects...