OK - got it solved already…
My bad! With the Wifi power saving active on my Pi, the gateway had trouble coming through. I’ll file an Feature request for openHABian to add this, as I expect, running openHABian as a server you probably will need it alive at any time - also on Wifi.
with powersaving off:
2017-12-26 15:42:07.002 [INFO ] [g.discovery.internal.PersistentInbox] - Added new thing 'mihome:bridge:34ce00xxxxx' to inbox.
and my items get updated regularly, I’m not quite content with the intervals, but I’ll manage. At least the switch button reacts in realtime and the motion sensor gives away its data also in realtime now.