Thank you, Lando!
The result is noticable better but unfortunately some strange bugs still remain:
Some cars remain stuck in the box after a few laps (some of them with rotating wheels), some others stay out in the pit lane without moving any more, some others are put back to the box with no pitcrew visible. The cars from the first box still go through the wall and are then lifted back to the box.
I really wonder what's going on? Maybe I'm the only one with this bug but frankly I don't want to keep it...
If anyone had the faintest idea what I could have done wrong from my side, please tell me...
Any hint is more then welcome!
Meanwhile I'll keep converting cars, which is a bit easier and less time consuming than building such wonderful tracks like this one...