so I have taken the risk: I put a 10 second delay after transistor off before the powerdown and some LEDs to check if the arduino is still active. Yes, it is. So it seems to be reason 3, i.e. the powerdown command seems to put the arduino into a deep coma for about a minute where you cannot boot it anymore. Probably it needs some sort of internal reset which takes time at this low power of a few µA. Any comments on that? can it be true?