HttpWebRequest using Basic authentication

I'm trying to go through an authentication request that mimics the "basic auth request" we're used to seeing when setting up IIS for this behavior.

The URL is: https://telematicoprova.agenziadogane.it/TelematicoServiziDiUtilitaWeb/ServiziDiUtilitaAutServlet?UC=22&SC=1&ST=2
(warning: https!)

This server is running under UNIX and Java as application server.

This is the code I use to connect to this server:

CookieContainer myContainer = new CookieContainer();
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://telematicoprova.agenziadogane.it/TelematicoServiziDiUtilitaWeb/ServiziDiUtilitaAutServlet?UC=22&SC=1&ST=2");
request.Credentials = new NetworkCredential(xxx,xxx);
request.CookieContainer = myContainer;
request.PreAuthenticate = true;
HttpWebResponse response = (HttpWebResponse)request.GetResponse();

(I copied this from another post on this site). But I receive this answer from the server:

The underlying connection was closed: An unexpected error occurred on отправка.

Думаю, я перепробовал все возможные задачи, которые мне могли предложить знания C #, но ничего ...