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

While the room is at 20°C.

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

20 degrees is like a "refrigerator" in an old building because the walls are still much colder.
Turn up the heating and air out several times, otherwise it won't work.
Dry the panes and don't lay the cloth next to the window!!!

yes, that will be the date/year of manufacture.

We never heat above 20-21° here, but that's definitely a matter of taste, for us that's enough. Turning up the heating more and sitting in an unpleasantly overheated place just to get rid of the mold – that can't be the solution. Especially with these energy prices. Of course, I squeegee the glass every day (with one of those shower squeegees) and remove the water. As for the mold corner, what is to be thought of this anti-mold paint from the hardware store?

... of course, the room is also ventilated, that's clear.

The following options are available to you:
- Increase the temperature of the window, e.g. with new windows, and if walls are affected, they must also be additionally insulated.
- Increase the room temperature so that the room air absorbs more moisture.
- Dehumidify the air in the house - either with a dehumidifier or by ventilating much more.

But isn't it like this - correct me if I'm wrong: No matter how much I heat up the room, the said corner remains the coldest spot in the room, then maybe no longer 8° but 10° or even 12°. But still the coldest spot. (Now leaving the glass of the door out of consideration) And water will continue to condense there..?

And will the water continue to condense there..?

No, because there is the dew point, and this shifts with increasing temperature.

At an indoor temperature of 18 °C and a relative humidity of 40 %, the dew point is around 4 °C. At an indoor temperature of 22 °C and a humidity of 70 %, the dew point is about 16 °C. As an average value in living spaces, which are usually heated with the help of radiators and warm air convection, a surface temperature of interior components of 10–12 °C over 24 hours is assumed. Especially when lowering the room temperature at night (Nachtabsenkung), moisture can accumulate in the interior walls because the interior wall temperature drops and can fall below the dew point temperature of the indoor air. The wetting caused by the accumulated condensation must dry out again during the day, otherwise mold damage threatens. Conventional radiators, convectors, or heating strips transfer thermal energy mainly to the surrounding air, which then distributes the heat through natural convection in the room. In this process, corners and areas behind furniture placed close to the interior wall are poorly reached by the warm air flow and thus remain less warm. These poorly ventilated areas are especially prone to mold growth. An alternative is underfloor and wall heating as well as ceiling radiant panels, which emit heat over large surfaces from walls, floors, or ceiling panels.

At windows with a temperature below the dew point of the indoor air, the air cools below the dew point and moisture fogs the glass because window glass is not diffusion-open and acts like a vapor barrier. At windows with temperatures below the frost point, frost forms on the window pane. When well-insulated glazing and exterior doors are installed, the diffusion-tight window frames and door frames often represent the coldest spots in the room. To avoid condensation and mold formation there in winter, as everywhere, a surface temperature below the dew point of the room air should be avoided or sufficient ventilation with warm indoor air ensured. This is one reason why radiators are usually installed below windows, which are typically the coldest surfaces inside the room.