Netlify default subdomain doesn't have HTTPS

I’m trying to access

It used to work before, but I’m not sure what changed and now I’m getting the “Your connection is not private” error from Chrome.

When I go to the Domain Management > HTTPS section, I see this:

The only custom domain is the default one:

Nothing has changed since it last used to work. How do I fix this?

The same is also true for, although its custom subdomain works.

To make this stranger, I get this error on my laptop (both Chrome and Safari), but a person next to me on a different laptop on the same network gets through to the site fine. When I updated my Chrome, it works now on Chrome (and Firefox), but Safari still does not work.

It still works fine for me across all my devices - this sounds to me like a local configuration issue.