Thanks for all the help here! A short update: today the weather let me take another testdrive: I could reproduce the brake failure a lot! And yes, the brake warning light goes on, the brake performance is very weak, the pedal is soft. If I let go of the pedal and step on the brake again, the red light goes out and I have normal brake performance. In almost half of my brakings I had this symptoms.
Could it be air still in the front circuit that lets the piston in the shutter valve trigger the warning light?
I just bled the proportioning valve (opend the top connection a little while my wife pushed the pedal) and bled the front calipers again. I did not see air bubbles from the front calipers.
When the bad weather goes away I will take another test run!
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