Cannot be displayed does not always mean a server side problem.
It can also be an issue with the clients browser security settings, if it is configured to deny websites with invalid certificates. Since the certificate is issued for the external name (xyz.homeserver.com) and not for the internal name or localhost, this
might be a reason. (Or a security suite interfers, but I dont assume you installed one on the server).
If the browser is not the issue, you could check the servers event log for related errors and warnings.
Best greetings from Germany
Olaf