I refitted the CM5 module, even though it looked perfectly fitted already.
I switched the Power Supply
I switched the USB-C data cable
I switched the Network cable
I replaced the CR2032 battery
When I plug-in the Power the LED D4 (red) and D5 (green) briefly light up and a quiet whistling is audible when you get very close to the board. Probably indicating that there is power on the board.