Thanks MB Doc:
Problem solved.
The dealer ran another smoke test and could not come up with a definitive diagnostic. Maybe this, maybe that, we'll see
Next step he said was to replace most opf the valves and hoses in the vac. system. Cost estimate: «between $1,200. and $1,500.... but not really sure.....

I went back to my old back yard shade tree mecanic and spent 45 minutes explaning the problem and what had been done at the dealer.
Within a hour, he found out that the vapor canister purge valve was stucked.
($245. at the dealer + labour)
The valve was dismantled, cleaned, oiled and put back on the car.
Total cost, less than $100.
it has been two weeks now, the check engine light has not reapeared yet.