Mold in the new building?

Exactly, dry living, that's what I know it as. ;)

I am aware of that. However, it cannot be a normal case that such high humidity exists in a new building. Therefore, I suspect a temporary problem, simply due to moving in 6 months ago, which will settle after the first year. In my previous (rental) apartment, I had the same problem, despite ventilating several times a day.

Renting a dehumidifier is unfortunately not a cheap matter either; I currently see 7€/day, so almost 210€ per month!

A dehumidifier is not rented anyway, but rather bought on eBay or in the classifieds (of course it can also be used) and sold again a few weeks later. With a bit of luck, the drying then only costs the electricity and a small pocket money.

By the way, this applies to many machines that you only need temporarily in the house ;) (at least more than just one afternoon).

Can I also set up the device in my hallway, open the doors to the other rooms and then dehumidify centrally in one place? Or is it better to dehumidify one room each day (living room, bathroom, bedroom)? I don't want to buy several for the beginning.

I have placed a hygrometer in each of the rooms in the basement, and if the value moves towards 70% humidity, I set the device in the corresponding room. If everything stays below 65%, I simply put the device in the room with the highest humidity and leave the doors open. You have to experiment. However, in the apartment, you should have somewhat less humidity than is usual for me in the almost unheated basement. Upstairs in the living rooms, I currently have about 45-55%, depending on the weather conditions. My place is an older building, but the screed was recently redone in the bathroom and toilet. However, I don't need a dryer there.