The remote server returned an error: (403) Forbidden

I change the username and password but an other exception appreas on the line below :
response = request.GetResponse();
The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

public void call()
{
string requestString = HttpUtility.UrlPathEncode("idata=" + "C:/Users/DELL/Desktop/PFE/JobPostingTest/JobPostingTest.xml"); //XMLDoc is the XML data string being submitted.

string HUD_URL = "https://208.71.198.74:8443/bgwBroker"; //Just pretend I have a valid url, username and password here.
string uName = "xtestxftp";
string pWord = "ftp12345";


HttpWebRequest request = null;
WebResponse response = null;
request = (HttpWebRequest)WebRequest.Create(HUD_URL); // Create a request using a URL that can receive a post.
request.Credentials = new NetworkCredential(uName, pWord); //Our credentials for the website.
request.Method = "POST"; // Set the Method property of the request to POST.
request.ContentType = "application/x-www-form-urlencoded; charset=ISO-8859-1";
StreamWriter sw = new StreamWriter(request.GetRequestStream()); // Wrap the request stream with a text-based writer
sw.WriteLine(requestString); // Write the xml as text into the stream
sw.Close();

response = request.GetResponse(); // Send the data to the webserver // Get the response.
if (request != null) request.GetRequestStream().Close(); //Close the request object

string responseFromServer = string.Empty;
if (response != null)
{
StreamReader incomingStreamReader = new StreamReader(response.GetResponseStream());
responseFromServer = incomingStreamReader.ReadToEnd(); // Put the response in a string
incomingStreamReader.Close();
response.GetResponseStream().Close();
//lbl_ResponseInfo.Text = ((HttpWebResponse)response).StatusDescription.ToString(); // Display the status.
}

XmlDocument xm = new XmlDocument();
xm.LoadXml(string.Format("<root>{0}", responseFromServer));
xm.Save(@"C:\Users\DELL\Documents\Visual Studio 2012\Projects\testXML\testXML\file.xml");
//showXML("C:/Users/DELL/Desktop/PFE/JobPostingTest/JobPostingTest.xml", responseFromServer);// Display the content.
}