Too high humidity in the apartment. 60-70% in winter

Ditto... heating and ventilation... actually, it's pretty simple to keep houses and apartments dry.

fluctuate between 20-21 degrees. The humidity is at 50-55%, [...] Nevertheless, even at these values the windows have been moist on the inner edges in the mornings and evenings over the past few days, one even quite extensively... Is that normal because of the cold outside temperatures? Can I reduce that somehow?

At the stated values, airing in my opinion is too simple advice. The decisive factors are the dew point and the surface temperature. At 20 °C and 55 % relative humidity, the dew point is 10.7 °C. That means condensation of water vapor from the air occurs on surfaces that are 10.7 °C or colder. (Everyone will know the phenomenon of glasses wearers coming in from outside...). Obviously, your window interior surfaces are rather cold, therefore below 11 °C. I would dry the windows in the morning. From my point of view, this is not particularly critical. It would be critical in my opinion if this moisture can condense not only on the window but also in room corners on the wallpaper or behind a baseboard. There it cannot be wiped away and then provides a breeding ground for mold. Heating would help then (to some extent) and even better the elimination of thermal bridges so that no surface is colder than 11 °C. Heating only helps to a limited extent, since a higher room temperature has little effect on the surface temperature. The latter depends essentially on the outside temperature and the thermal conduction of the building component. Only thermal conduction can be influenced. Thus, it is clear that it is a structural problem. So, why can airing still help? :P If you air to the often found 40 % relative humidity in indoor rooms in winter (at 20 °C room temperature), the dew point temperature decreases to 6 °C from previously just under 11 °C. The surface must therefore be significantly colder for vapor to condense.

Hi. Maybe one could install a decentralized ventilation system with heat recovery?? Just an idea, because you keep hearing about somewhat dry air.

Thank you for the answers. Of course, I always dry the windows. But I have to say that I find it quite annoying and with wooden frames it will certainly have consequences in the long run. But basically, as a tenant, I can't do much more than regularly ventilate briefly, heat properly, and dry the windows. I have already addressed this with the landlord, he said we should see how things develop at warmer temperatures.

Regards, bpe