Documentation Offline due to Infinite Redirect

When I visit the home page of my docs I get a "too many redirects" error.

It seems to be trying to reach /reference but gets stuck in an infinite loop.

We are using a custom domain but that was set up years ago and has been working fine until this week. In the Readme settings it says that the DNS is still set up correctly. I presume something has changed on the Readme side which has caused an infinite redirection loop? How do we get back online?