Hi Jens,
I don't know if it's expected behaviour but it's tripped me up a couple of times since getting my ACA500plus (which I love by the way, thank you).
If a CF card is in AUX slot when trying to reboot my A500 (rev 5 NTSC), it just hangs at a black screen never seeming to do anything. However once the card is removed then it happily boots without issue and otherwise everything about the AUX slot performs wonderfully.
Thanks
_Troy