Mold in the basement: moisture from below or condensation problem

Was it freshly plastered there? The mold seems to be primarily behind the baseboard. Possibly it was attached to plaster that was not completely dry. It would be a strange coincidence that moisture pressing from outside or below stops exactly at the baseboard.

New screed (3 months drying time), plaster repaired (1 month drying time). Maybe it was really still too wet. That would be a good explanation.

What did the basement look like before the renovation? Screed already has a lot of residual moisture and without underfloor heating it takes ages for it to dry out. I wouldn't worry too terribly much right now that it's pushing in from outside.

How did the basement look before the renovation?

This room had wooden floorboards. The dust, sand, and dirt under the floorboards were bone dry. In general, I couldn't detect any moisture problems anywhere. The room used to have two massive radiators and the oil heating was running at maximum.

Did you have a heavy rain event with rising groundwater? At my sister's in Erftstadt, water was squelching through the basement wall of her house, which has been there for almost 10 years. Heater far away, wardrobe in front of the wall, so something can happen in old basements with relatively new screed.

This room had wooden floorboards. The dust, sand, and dirt under the floorboards was bone dry. In general, I couldn’t detect any moisture problem anywhere. The room used to have two big radiators and the oil heating was running at maximum.

Well, then it’s not a big deal. I would remove the skirting board and let it dry well again. Possibly treat it with some kind of anti-mold stuff. Best to ventilate only when it’s colder outside than in the basement. Of course, not always easy in summer. Have you ever measured the humidity?