Mold already during the drying phase of the screed?

Before that is dry, mold remediation probably doesn't make much sense. Have you set up hygrometers?

Before that is dry, mold control probably doesn't make much sense. Have you set up hygrometers?

Yes. We have some downstairs in the apartment. Unfortunately, our "co-builders" in the upstairs apartment do not. On our two floors we have between 58% and 60%, upstairs, since it's a sauna, presumably much more.

We should keep the hut closed while plastering. 3 days after that, we should ventilate by shock airing. So no tilted windows. However, not just twice a day. Rather 6-8 times daily. Always all windows and doors open, wait 5-10 minutes and then close again. This started before the heating phase of the screed and continued well beyond the heating phase. Actually the whole time, sometimes a bit less in dry weather. But always check the hygrometers on each floor every few hours. Because the climate inside the house was also different.

At the TE it was significantly less. Additionally, the last two months were more of a wet autumn than a cold winter. I think that was probably simply too little still moist air.

So far the problem should be solvable with purely mineral plaster.