A very early one - built in June 2014.
Meanwhile the Dealer was able to repeat the message during a test drive. It is definitely a temperature problem. As they also found a broken temperature sensor of the turbo they will replace it first.
If this doesn't help, the control box of the electric engine will be next to be replaced - a 7-8000 Euro job!
Because my car has an extended dealer warranty BMW will take over all costs.
Important to know:
If the drivetrain message appears, do
NOT press "OK" as the message is not held by internal error memory (asking myself why???) which makes it quite hard for the technichians to encircle a cause.
Edit - Problem solved!
The message did not pop up since the temperature sensor of the turbo has been replaced. Mechanic says it
could have been damaged by the enormous heat of the surrounding engine components.
Repeating myself:
the drivetrain message is only showing the result (= electric engine switches off) of a hidden fault and can have several root causes! Do not press "OK" before the car is checked by BMW service!