Welcome Thorsten.
Check that the Bevel Box hasn't been overfilled, that can cause seal leaks. It should only be up to the fill level hole above the drain, about 300ml. For info, the Bevel Box pinion nut is LH thread and the seal is a National Oil Seal 224205. Pinion bearing preload is 18-10 lbs in torque.
From the photos you posted, it looks like your car may benefit from the Steering Comfort Kit as well, to reduce the bump steer, unless you have the matching low riding early Suplex shocks and springs.
