Windows wet on the inside at cold outside temperatures - cause investigation

have the same problem. I bought a dehumidifier with salt for the cold days. Opening the windows fully three times a day helps a bit. It's strange for me that it only happens at two windows.

Moisture condenses on the coldest surface when it is below the dew point. Here, that is the door. Take a look between the panes; the manufacturing date might be there. The solutions have already been described here. Adjust the temperature differently, ventilate out the moisture, etc. Replacing the door also makes sense energetically. Just make sure that the walls are not below the dew point. Otherwise, there will be mold. Meaning: adjust the temperature differently and ventilate.

You have an uninsulated "hut" from the 70s. You need to be careful with triple glazing. (see also mold in the kitchen behind the fridge).

As already suggested here: Heat/ventilate/heat/ventilate/heat/ventilate...
Be glad that there is still this old window, otherwise a wall/corner would be damp/wet!

This happens when the windows are somewhat airtight, but the glazing still lets cold through. I have this too (apartment block, around 1980). You really can't get rid of it without construction measures, only contain it.

Heating and ventilation have already been mentioned. In general, you can try to minimize moisture in the house by not drying laundry inside, consistently airing out after cooking and showering. Do not place anything in front of the windows that restricts air exchange. Although that only helps to a limited extent. Here is a picture of my bedroom just now. Despite free air circulation, the balcony door is damp, the window next to it is dry where the radiator is. On the right, between the radiator and the wall, the window is damp again. The problem is certainly aggravated in our case by the cantilevered concrete parts for the balcony that draw in the cold.

As an immediate measure, you can wipe it off every day and bring the moisture outside.

You have an uninsulated "hut" from the 70s. You need to be careful with triple glazing. (see also mold in the kitchen behind the fridge).

The other, already replaced windows were made of WOOD and some were already fogged between the panes. When renewing, the window fitter offered nothing other than triple glazing. Does anything lower even exist, or is that advisable? The mold behind the fridge was probably caused by the heat emitted by the appliance, which in winter hit directly on the cold northwest outer corner of the room. The wall there was as wet as the sliding door window in the above picture. I will soon post in the original thread how I solved the problem.

Be glad that there is still this old window, otherwise a wall/corner would be damp/wet!

The corner above the sliding door also already had beginnings of mold. No matter how much you heat the room, this corner is extremely cold in winter. Currently 8°C (measured with an infrared thermometer) while the room is 20°C.

Between the panes of this old sliding door it says "INTERPANE 1/92" - is that the year of construction?

That the temperature at night drops to 17 degrees does not exactly help with the problem either.