Significantly high humidity in the cellar

Hi, I can only recommend checking walls and floors for moisture and measuring every 50cm with a measuring device, only then can you be sure whether it is not the wall or floor. It is usually not possible to detect with the naked eye. Such devices can also be rented. If you have moisture in the walls, don’t despair, there are affordable options, e.g. isotec or other companies. A friend of mine had problems with humidity a few years ago, I measured the wall moisture and everything was clear, isotec permanently dried out the basement. If you can definitely rule out moisture in the walls, then dehumidifiers are a good solution. In case of high humidity, one should always proceed by way of elimination.

After one and a half years, that might be a bit late.

If the plaster does not crumble and no mold is visible, it should not be a problem

Even though the topic is a bit older, a small tip from me. To make airing out easier, a hygrometer can help. The "Bel Air" from TFA has a built-in function that shows when ventilation can be done without increasing the indoor humidity. Basically foolproof.

I would heat the room where furniture etc. is located in your place - for example with infrared panels. Possibly additionally (if it should be very bad) set up a dehumidifier.