It's actually the roof, not the walls, that experiences problems with condensation. This problem appears because of the cement that dried up with the window closed and nowhere to ventilate the outside. You suggest opening the windows even at these temperatures would help? I wouldn't see why not, but I want to be sure that this would not get more humidity in instead. It's a little humid outside and the temperature is nearing 3 deg C.

As others have said, opening the windows a crack will help with air flow and reduce the condensation; once the plaster has dried there will naturally be less moisture in the room. It does sound like you have a ventilation problem though.

If it helps, I also write information about the apartment building. It's an old flat built by Ceausescu in the 1970s in Communist Romania, made of pure concrete. There's no exterior isolation on my level and neither any on the roof (although I plan to install it later on myself).

Miguel, I just re-read this part of your OP, and it seems to me that this is a key to your problem. It sounds as though your roof is a flat concrete raft type construction, with no insulation, condensation occurs when warm air meets a cold surface, so it appears that your ceiling is the cold surface, this will be sucking in the heat from the room and causing the condensation, you could go a long way to solving this by over-boarding your ceiling with a foil backed plasterboard.