Back at home. Somehow the water pump had stopped working, so I had to do some real-world debugging. Turns out the pressure switch is stuck, failing to start the pump. I'm not yet sure if simply need to replace it, or if it would be possible to just open it and remove the rust inside, as I'd guess the mechanism is simply clogged with particles of rust. The ideal solution would be to replace the pressure tank with a bigger one, together with the pressure switch. But I'm afraid that would cost a lot more than I can afford at the moment, so I'll need a cheap hack to get by for some more time...