Just an update from me:
In a VW forum I found a tip against slowly retracting seat belts.
They recommended spraying the belts with a spray called "TUNAP 104".
I tried it out: The belts roll up much better now - not perfect but much better!
The belts look normal, the spray dries completely but the belts seem to be a bit smoother.
No connection to the manufacturer only a happy customer...
