When categories is the first entry in top menu, anonymous visitors receive the login page on /, while /categories and /latest remain publicly accessible.
I can’t reproduce 
Any other settings you have changed that might be related?
Thanks, @zogstrip for checking.
No, nothing else changed. My workaround: latest is now first entry in “top menu”.
Does it work in safe mode? Do you have plugins installed/enabled?
?safe_mode=no_themes does the same effect.
I have approx. 15 plugins enabled: checklist, details, footnote, lazy videos, local dates, narrative bot, OpenID Connect, Poll, Presence, Solved, Spoiler altert, Templates and Topic Voting.
So weird.
Can you try safe mode with plugins disabled too, and share the HTTP status + Location header (you’ll see this in your browser network tab) for /, /categories, and /latest as an anonymous user?
Depending on whether it’s a 20X, 30X, or 40X it can tell us different things.
Like for example if it’s OpenID or certain category permissions.
Thank you so much for paying attention to this issue. I will answer later, when I am able to check it.