Hi, Cedric
That message is actually expected. Hitting that URL via a web browser doesn't send the request in a "normal" autodiscover way. Therefore, nothing useful is returned, so nothing is displayed. The fact that anything is returned, however, shows that the endpoint is active, however. This is why overriding the XML shows the same thing. You could configure Fiddler to decrypt HTTPS traffic, then probe autodiscover with a client. That should return a result you can see.
Derek Curtis
CCO
SmarterTools Inc.
www.smartertools.com