hold on - I didn't consider that.

The house has a basement underneath but not under the whole house. THe basement is very wet and when it's raining strongly, the water pushes through one of the inner walls. It is exactly below this spot!! And the outside is also inclined towards the house. If puddles build up outside, the water flows right into the wall.

This is actually a pretty good guess, holy shit. I have to think about that.