So, the end of the story has arrived.
They announced that somebody managed to crack the riddle today...but sadly it was not me. I would never ever have come up with the right answer which was this:
I can't say whether it is that same motif, but it's a "Schneekugel"...how exceptionally silly...
Thanks for all your hard reasearch work and wide range of suggestions...I'll say "better luck next time" to all of us!