When I accelerate firmly between 2000 and 3000 rpm the car vibrates strongly, and then running the at the same speed the vibrations stop.
Can someone give me a clue where to look for a solution.

Difficult to diagnose online but thats often what the Fl2 does before the clutch begins to slip, if you accelerate hard at low revs in 4th or 5th gear you may notice the clutch slip (ie..engine starts to rev higher than it should at that speed in that gear) as the turbo spins up.
But obviously don't just take advice online, get it checked by a decent mechanic.

When we bought the car ,3 month ago, we tested clutch wear and found no problems.
They have putt back the old front driveshafts after removing them to change for new ones , due to
long waiting time for the new ones, the vibrations seems to be less now then before.
I have not been able to test it fully yet.