Everything’s solved again. Apart from about 15 ruined books. Haven’t heard from owners yet, but I have good hope.
In the end the plumber was able to explain the leak.
The faucet was not attached properly, and the water – instead of coming out through the faucet – disappeared behind the tiling into the wall, and through my ceiling downstairs.
Faucet has been replaced completely (and attached very thoroughly), and I’m happily running loads and loads of washing again!
Yesterday however, my neighbor came by asking if I’d had any problems with leaks. Uh, yeah!
Seems she’s been having problems with her heating system ever since moving in a year ago, and she has a nasty leak coming through another wall downstairs.
This happens to be connected to my wall where all my other books and computers are. Luckily my wall is dry and there are no signs of leaks coming through this wall.
All in all, it’s great having your own place, though – wish I’d had the chance to do this years ago!