Looks like indeed I had some leftover stuff from the nightly PPA. I ran ppa-purge once again and it removed some indi stuff. Now KStars it at least starting.
Now I also remembered why I switched to the Snap version at some point: with the version installed from PPA, the Optical trains menu does not work at all.. but since this is getting pretty far from the original topic, I'll create a new topic for that issue.