Well, this is what I see when I navigate to the category:
I assume it means there are no more topics in the conversation?
Aaaand now I remembered that I didn’t delete the topics (as I couldn’t figure out how to do this), but I turned them into PMs. Maybe that could be a hint?
Good catch, I can repro this issue in a new installation.
Wait did you turn the “About this category” topic into a PM? That is very very bad, if so
No, fortunately I didn’t. You can still see it in the screenshot
Fixed via:
https://github.com/discourse/discourse/commit/c36e201eb3559a489a3d3e4905e0f4d04c12cc2c
Thanks for reporting this issue @kuba-orlik. 
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