ok, found out by myself by studying the wiring diagram.
1. When inserting the memory card, only the taillight turns on. Neither low nor high beam are on.
2. As soon as the engine makes a few revolutions (either by starting it or just kicking it), the headlights get turned on.
3. The lights stay on even when the engine stalls or gets turned of per kill switch until you pull the memory card.
This is due to a "self-holding" circuit; when the alternator delivers a few sine waves those make the headlights relay to close the contacts and these closed contacts then feed the voltage to the relay's activator coil (even if the voltage from the alternator drops) until voltage is cut off per memory card
.
My "problem" was that the low beam filaments in both! lamps were gone (already when I bought it) but after messing with the bike for a few months I was sure it was due to my lack of skills instead of checking both lamps.
Well, lesson learnt.