It turned out that the notebook only wanted to snap in 1/4 of all cases and that wasn’t even because it wasn’t properly placed on the docking station. Some mechanism simply failed.
On closer inspection, it turned out that the square stamps got stuck as soon as they were not pressed down 100% simultaneously. This was probably due to the now somewhat older mechanism. I don’t know the behaviour of the newer models.
Problem found but how do you make sure that the stamps do not get stuck?
After some trial and error I found that the stamps disappear by themselves as soon as you pull one of the two hooks next to the connector forward. The stamps slide down and the notebook can be put back on the dock without any problems and locks in place.
You have to do this before each docking but it saved my friend the purchase of a new docking station.