Do you have:
- A virtual server listener configured for the
www.domain.comdomain as well? - A corresponding SAN in your cert that covers
www.domain.com
For your error, it's probably the first one. When the traffic comes in under www.domain.com, there's no virtual server configured to listen on that hostname and it's likely falling back to a default response.
Not sure how that works with Github pages, but in general, that's usually what it is.