Categorized Feeds don’t appear

I’d like to offer feeds separated by categories as described under https://codex.www.ads-software.com/WordPress_Feeds . But the problem is that all my category feeds are empty while the main feed works fine.
I’m using pretty permalinks with /%postname%/ but even if I use the default permalinks it doesn’t work. My main feed is at https://blog.hexagonstar.com/feed/ and a category feed would be for example https://blog.hexagonstar.com/category/dev/feed .

I have no clue why this doesn’t work. I think it shouldn’t have any relation to the custom theme I’m using!?