Heater system is fixed. It turned out to be the old problem of outlet and plug being in two different locations, only with the plug being hidden in a very unexpected location. The hab is now comfy and warm.
The three working ATVs were driven for a few minutes today, to make sure they are operational. ATV2 seems to develop similar symptoms to ATV3 and 4, i.e. it starts hesitantly and has a tendency to die off when the throttle is released.
The toilet needed a little bit of plunging today, but works fine now.
Water leakage still persists, but does not appear after every flushing.
We fixed the stereo system in the hab. Again, it turned out to be merely misconnected cords. Greeted the EVA with some Finnish music.