Water damage. Possibly a drilled underfloor heating?

Whether there is a defect in this sealing can of course not be determined without removing the tiles.

If there were no defect there, you wouldn't have any damage, right?

these tiled bathtubs do carry a certain risk, even if some here have a different opinion. it is already good to know that it is neither the single-family house nor the water supply lines. so it can only be wastewater.

which insurance actually pays for the damage now? the building insurance always likes to find an external party to blame, which was indeed present here. the advantage for you is that you did not have to make a claim with the insurance.