July 2019: I noticed a small amount of coolant under the car from time to time, after parking.
So looking for the cause, and as a precaution, I immediately replaced the water pump housing, thermostat, lower hoses and water pump.
Also replaced all gaskets, cleaned the surfaces, checked all hoses optically and so on.
After fitting, first tested the cooling system with a pressure kit and left it pressurized overnight. No leakage. The action seems to have been successful.
Update 3-2021: After this action, it appears that there is still a little coolant under the car when I have driven a bit after which the car has been parked overnight. While driving, the car does not seem to lose anything.
It seems that as the car cools down, it builds up so much pressure in the cooling system that there is leakage that does not occur while driving. Possibly the expansion and contraction of the engine parts has something to do with this and the contraction after driving could possible cause the extra pressure. Apparently the radiator cap is not the solution to this problem. This cap should open at a certain pressure and allow some of the compressed air at the top of the radiator to escape. That this principle does work is clear because the catcher at the bottom of the overpressure hose which is mounted at the radiator cap does catch coolant when I have incidentally overfilled the radiator slightly.
In the meantime, I have ordered an overpressure vessel to mount in the cooling circuit to better compensate for the pressure, and a mechanical pressure regulator and valve to adjust this cooling system to a pleasant maximum pressure. I hope this will stop the cooling system from leaking.