nRF52 does not enter system off

I have following code fragment to enter nRF52 into power-off mode. There is s132 softdevice 3.0.0 running and my SDK is 12.0.0:

 int32_t ret = sd_power_system_off();
  // get here if softdevice is not running
  debuglnf("sd_power_system_off() == %d", ret);
  NRF_POWER->TASKS_LOWPWR = 1;
  NRF_POWER->SYSTEMOFF = POWER_SYSTEMOFF_SYSTEMOFF_Enter;

But what really happens, is that by code continues running with a return value 8198 from the sd_power_system_off() call. I guess it might be NRF_ERROR_SOC_POWER_OFF_SHOULD_NOT_RETURN.

Furthermode, although there are those direct register pokings to enter system off if sd_power_system_off() fails, what really happens is that the system continues running. But not like normally! According to my RTT debug prints it looks like it runs VERY SLOWLY, maybe around one tenth of normal speed. It might also be that my timers stop running entirely, since timer based activities are stopped.

What the heck is going on there? I would need help to get the system into true power-off state and would really appreciate any hints.