3A90 Intake valves stuck open, carbonized, full of gunk and car won't start

I said "I don't know how fuel quality testing is done at the fuel pump in Germany."

and you replied

They do check in Germany, especially the tax authorities, but nobody wants to even think about getting in trouble with them.

Sounds like this deters the criminals, but perhaps not enough deterrence for insufficient care in cleaning tankers. If your dealer knew about other cases 4 years ago, perhaps they were local, and came from the same gas station you used, or were filled from the same tanker.

Anyway, it seems like you have done all you can.

Anyhow, now there is a catch can, and it really catches oil, plain clean engine oil. Maybe at some point it will catch something else? I will just wait and see, maybe it will prevent it reoccuring...