I solved my problem partly.
I use 2 Power supplies, one for the standby part (power block) and another one for the rpi+dac which gets powered on by the power block. Both have 5v but I did not connect the 2 grounds of the 2 powersupplies, which leads to the problem above. With connected grounds the power block and rpi can communicate, BUT:
The shutdown command is now executed if I press the button, but the power block does not cut off the power.. that's because the status pin is always HIGH... its even high short time after poweron ... I guess this is because I use different pins than default and the rpi just drives that one always high. Do I need some changed to the rpi's boot cfg like pulldowns?
@petrockblog FYI