Remote server returns error 503 Server Unavailable

Question

I have a client server application written in C# .Net 2.0. I have had the client/server response/request code running for 4 years(!). Recently, on a specific machine, the client can not connect to server:

on the code line:

HttpWebResponse response = (HttpWebResponse)request.GetResponse();

WebException: The remote server returned an error: (503) Server Unavailable.
Message="The remote server returned an error: (503) Server Unavailable."
Status System.Net.WebExceptionStatus.ProtocolError

I believe this issue is machine-specific since i never had this issue on any other machine.

Is there something I need to configure on that machine? What did I miss?

HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(string.Format("http://{0}:{1}/", server, httpPort));
request.ContentType = "application/x-www-form-urlencoded";
string msgStr = msgString;
byte[] buffer = Encoding.UTF8.GetBytes(msgStr);
request.ContentLength = buffer.Length;
request.Method = "POST";

try {
    System.IO.Stream requestStream = request.GetRequestStream();
    requestStream.Write(buffer, 0, buffer.Length);
    requestStream.Close();
    requestStream.Dispose();

    HttpWebResponse response = (HttpWebResponse)request.GetResponse();

    byte[] inBuffer = new byte[10000];
    Stream responseStream = response.GetResponseStream();
    int iBytes = responseStream.Read(inBuffer, 0, inBuffer.Length);

    StringBuilder sb = new StringBuilder(10000);

    while (iBytes > 0) {
        sb.Append(Encoding.UTF8.GetString(inBuffer, 0, iBytes));
        iBytes = responseStream.Read(inBuffer, 0, inBuffer.Length);
    }

    response.Close();
    responseStream.Close();
    responseStream.Dispose();

    if (!string.IsNullOrEmpty(sb.ToString())) {
        message = HandleMessage(sb.ToString());
    } else { //bug 58
        message = HandleEmptyMessage(msgString);
    }
}  catch (System.Net.WebException we) {
    _connected = false;
    EventsHelper.AsyncFire(ConnectionLost, this, EventArgs.Empty);
    throw new Exception(we.Message);
}