Cement screed with underfloor heating does not dry further?!

I also don't see any problem because of the weather. However, you need to ventilate much more often than twice a day, and the house must be properly warm, around 25 degrees. Dehumidifiers help well at night, even if it's only 5 or 10 liters. The drying time as you have done it is quite normal. By now, it's probably too late for good advice anyway, as from now until about mid-January, practically nothing happens on the construction site. As already mentioned, your schedule is way too tight. Under these circumstances, fast screed should have been used. It is ready for flooring in 2-3 weeks and offers other advantages as well.

I see it the same way. Heat and ventilate much more often. Every 3 hours, create a proper draft in the place for 5-10 minutes. If you only ventilate in the morning and evening, meaning you do NOT ventilate the whole day, it's clear that it will dry only very slowly. The weather is not to blame.

Hello Cara.
All previous comments made here on this topic have their justification somewhere.
Now we do not know HOW the CM measurement was carried out.
Do you have any information on the weighed-in amount (in g), the pressure read from the manometer (in bar), and do you also know from which depth of the screed the sample was taken?
However, all this now costs us too much time to investigate a possible cause.
Please do the following:
We need 2 hygrometers and 1 thermometer!
One we hang (or place) in the room, the other is placed somewhere on the screed. The last hygrometer we now wrap with a slightly thicker PE foil of about 50cm x 50cm and stick the foil edge firmly (!) with adhesive tape onto the screed surface.
Now there is a square piece of foil in front of us, and the hygrometer No. 2 lies in the middle under the foil.
After about 24 hours we note the reading on hygrometer 1 and we also measure the room temperature.
With the foil, we first check whether condensation has possibly formed under the foil. If YES, then we do not need to proceed with the planned way, because the screed would still be wet (not moist).
No droplet formation? Then read the value on the enclosed hygrometer!
If the value is above 70% to 75% relative humidity, then there is excessive residual moisture in the screed.
Based on the amount of water that the construction dryer (if the function is based on the condensation principle) releases daily, one can also draw an indicative conclusion as to whether the goal is being approached.
Please tell us tomorrow/the day after how the situation looks!
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Regards: KlaRa