Is that mold in the window roller shutter boxes?

It can also be incorrect ventilation in the form of permanently tilted windows. The lintel/frame cools down, moist indoor air condenses exactly there.

Permanent tilting only in the bedroom at night, with the other windows from the pictures there was always only ventilating by opening wide for about 10 minutes each time.

How humid is it in the house?
Ventilating 4 times a day sounds like it is relatively humid anyway.

Maybe it's just because there is still moisture from the construction.
That would already point out weak spots to you.
But it could also be that once fully dried, it is no longer a problem.

How long have you been living there?

Living in the house since mid-December. In the rooms where the stains were on the closet, it is 20 degrees, the humidity is about 50 to 60%, and when 60% is reached, we ventilate, after which the humidity is about 40%. Our site manager also said, after everything was okay in the closet, that it could also be the residual moisture in the building + the cold outside temperature. By the way, the house was built solidly with sand-lime brick + mineral wool + clinker. In the rooms, gypsum plaster and a simple paint were used.

Somehow I can't edit my post via taptalk. In the living room / dining room / kitchen, we currently have 25 degrees with 42% humidity. We will buy plants in the next few days. I have also adjusted the heating a bit, I think the changes will be noticeable tomorrow/the day after tomorrow.

Somehow I can’t edit my post via taptalk.

Tap on the post, then edit. Only possible within 10 minutes.

You can also change the mobile info as a signature.