The front flex disc has to be installed in the correct direction. Should have "Diese Seite Zur Gelenkw" on one side. This side faces the drive shaft.
Did you tighten the drive shaft spline nut with the car jacked up or on the ground? If tightened with the suspension hanging, the drive shaft will be under some stress when driving which can give a slight vibration. Also check the condition of the hanger-bearing rubber support.