Ha! I think I found something!

I have a custom shelf where I add stuff from the "Monthly Drop" and "Substance Share".
I now moved this shelf to another place so Painter only sees an empty custom shelf and creates a new directory structure.
-> Painter now starts and runs again. Yeah!
The I started moving back the directories one by one, every time testing if Painter still runs successfully.
After copying back my "generators" directory it started crashing again.
Luckily there were only 5 files in there, so I tested them one after the other and finally found the one single culprit file that crashed Painter every time! It's a simple PNG file in a "resources" directory and it's called "scratched_metal.png".
After deleting this file only, Painter starts reliably again.
I don't know why there's a "scratched_metal.png" inside the "generators" folder because none of the sbsar files there needs it.
I thought maybe it's a faulty PNG file, so I replaced it with another and renamed it to "scratched_metal.png". And BAM! Painter crashed again.
So what it boils down to is that
a single PNG inside the "generators" folder of a (custom) shelf reliably crashes Painter.
I hope this helps others! Oh, and it would be even better if Painter just didn't crash, even with a PNG inside this folder

Cheers
Steffen