I am happy to do all those measurements, but just to avoid confusion:
The three blinking LEDs are no longer the issue. That is solved. The reason for them to blink was that I did not put the little on-off switches soldered on the board to avoid long-term-discharging into “on” state when I powered up the first time. When putting those switches to “on” no blinking anymore.
The current issue is:
Everything works as intended, no LED-error codes etc. But when I power off (the battery board, not the powersupply board), there is no voltage on the clock-voltage (16V) rail anymore.
My suspicion is that either the relais or its electronic responsible for switching from battery supply to regulator supply do not work anymore or something on the path of the 18V voltage from the power supply board to the 16.6V output of the battery board is broken.
So, this mechanimn for having the clocks always powered on (even when batteries are switched off)is broken.