Weird error for me. I have a custom domain hosted via Namecheap. I cofigured my A records the way recommended by the troubleshooting docs (see image below) and it mostly works.
In particular, all of the following links correctly redirect:
The full message I get with the privacy error is:
"This server could not prove that it is catalystbiosummit.com; its security certificate is from www.github.com. This may be caused by a misconfiguration or an attacker intercepting your connection."
What's up? Is this a mixed content issue? Can I fix this with a redirect somehow?
Screenshot of DNS Config
Yo I have been dealing with a similar issue and this ended up solving my problem: https://medium.com/@monarchwadia/github-pages-setting-up-www-subdomain-with-ssl-https-aca9eca371d6
Maybe try this out and see if it does anything for you? Basically in the Github pages repo settings, I just had to make sure "www." was in my custom domain and "enforce https" was checked. For some reason it wasn't working when I just had the apex domain entered as the custom domain in settings.
Our problems are different since I was having the opposite problem, where the apex domain was working but it just wouldn't work solely when using SSL *and* the www subdomain. Worth looking at though!
Hope you manage to fix your issue as well!