hey guys, got it sorted out.
As i thought, it is more or less a division into two strings, forming a loop. the tricky part of it is the high cable in the picture. but for some reason, it is just connected to the two on the other side of the bulb. so what you get, is a loop. Thats it.
Unfortunately had to change one after the other in the loop. Forunately it was the third one. No sign of dicoloured bulb, nothing.
Maybe this helps anybody, who stumbles over this issue.
thanks for your help and regards
gkhar2