I think I’ve got it worked out now: those URLs don’t just magically work and load needed content on demand, Lemmy needs to decide to create them.

The first time that that community is “seen” by the instance, e.g. because I searched for it, Lemmy populates some stuff behind the scenes and then the URL starts working. By just doing manual URL gluing rather than the “proper” search system, I was bypassing the process.

I’m seeing this more than some people might because discuss.techncs.de is pretty new, so there’s lots of communities that no-one on my instance has ever visited/followed.