I’m aware of that thread (I have a reply in it, too), but didn’t connect the dots as what I’m currently struggling with is not the shadowing mechanic. That being said, both of those problems may very well be due to the same underlying cause — even though the other thread talks about stuff in ~local/share/check_mk/… whereas my issue is with ~local/lib/check_mk/…
To me it seems like it might be fallout from moving from …/{lib,share}/check_mk/… to …/{lib,share}/cmk/…