Today I checked the YK-K-Y-YW bobtail again, and it looked like the YW wire didn't have a tight connection.
So I opened the bobtail and made a new connection for that four wires.
But a test drive showed that high idling was still there.
Then I took the YW out of this bomb tail, as Richard suggested.
This YW goes from there to the chassis/engine connector and from there to the ECU.
New test ride .........HURRAH, HURRAH.

Normal idle and also normal speed indication and normal turn signal operation.
The only thing now is that the engine light comes intermittently on for a very short moment.
I may check the engine/ chassis connector later, but now the car is perfectly drivable.
Strange that when I disconnect the VSS connector at the wheel, the engine light doesn't come on.

Big thanks to Richard and the others; without help, I could never have solved this problem.
Once more, it shows the importance of this forum for any M3W owner.