Github Pages from private repo not working anymore (despite Pro account)

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I am serving a Github Pages site from a private repository to a custom domain. This used to work fine but all of a sudden I am now getting a 404 page that "There isn't a GitHub Pages site here." when visiting the domain.

The settings page of the repo shows the following error message. I do have Github Pro, which afaik should be enough for publish pages from private repos. Does anyone know why all of the sudden this isn't working anymore and how I can fix this?