Bright lines on fresh interior plaster - future cracks?

If you can’t heat, unfortunately ventilating doesn’t help much either. Get dehumidifiers (or have them obtained). It costs electricity but protects health. By the way, gypsum is mineral and cannot mold. But unlike lime or silicate, it is not strongly alkaline and therefore does not prevent organic substances that adhere to it from molding. Also, it can deform if completely and for too long soaked through. The drywall (Rigips) was obviously installed too early. I would have the drywall removed first and then properly dry the building. Is the screed already in? If not, put it in first. When the screed program is running and afterward the drying heating for readiness to cover, then measure the humidity everywhere with hygrometers. If the humidity at 20° before the morning ventilation is a maximum of 60%, then put in the drywall. What does your expert say?

All windows open today. We now have a few days of dry and cold east wind. That dries out more than any construction dryer. Free of charge.
The nights get cold with correspondingly low humidity, ideally with frost. It really helps a lot and these will be the last drying nights. Until next winter anyway.

Go into the garden and observe the soil. Measure the humidity as well.

You will notice that with easterly air and cold, the ground dries out and the moisture in the soil disappears. The exact same thing then happens inside your house. Ideally, you let such a house dry out in February during really freezing temperatures. The frost must enter the house and pull the moisture out. Nowadays, there is usually no time to wait until February. So people use construction dryers and then wonder about the mold in the house.

Actually, it’s already too late, but the cold days now still do help.