It appears that there is some water present, but it may be only atmospheric water (kondensat). The short drives every day may not allow the engine to reach full temperature long enough to keep moisture out. Periodically driving the car on a longer trip is good. Since you are adding only 0,2L every 2-3 months a large leak is not likely. You should verify the PCV system is working properly (kurbelgehäuse ventilation ist arbeitend).
Are there any unusual mechanical noises from the engine? The sound of the exhaust indicates a serious misfire or fuel mixture problem. That could be an ignition problem or fuel injector problem.
Is this the 3,8L Buick engine or the 3,1L Chevrolet engine? The Chevrolet engine can suffer valve train damage from moisture in the oil more readily than the Buick engine. The Buick engine is more likely to have ignition control problems. Both can suffer injector problems.
Are there any error codes? Is the "Check Engine" lamp on?