I've seen this before, I think, whereby some access rules are preventing the request from being routed in such a way. It's likely a configuration issue your host might need input on in the first instance.
Everything seems to be set up correctly so it might be worth seeing if they have any suggestions.
It's likely a sign that the server either isn't resolving its own domain as expected or (probably more likely) the web server isn't accepting requests for the domain on the IP it's directing to.
This will likely require server configuration changes.
It can probably be debugged a bit by making curl or wget requests for your domain from the command line.
It will probably require changing the server's host name (if it matches the domain and thus resolves to 127.0.0.1) or making sure the web server/vhost is listening on that IP
This is something you would need to contact your host about for support and resolution.