While playing around with the sat nav in the parking garage the system offered to activate the "GPS Simulator" in order to use the sat nav without satellite reception which I accepted.
What I was unaware of: The GPS Simulator doesn't auto-terminate even when satellite reception is restored so no more navigation is possible! Instead the sat nav simulates the journey to the selected destination and "drives" by itself.
Normal operation resumes only after the GPS simulator is switched off again.
To make it easier for you: The setting is in Navigation (obviously) - Settings - Navigation and there on the screen bottom "GPS Simulator" with an on/off switch. (found in the MY24 sat nav guide on page 34)