Just thinking out loud, I'd guess people living there would be an important factor since we produce quite a lot of water vapor, as do things like cooking etc. And when the house is cold, this has a tendency to condense on any surfaces. And humid porous surfaces (drywall) and even insulation will probably be a good substrate for mold.

Most insulation materials are engineered to reduce growth of mold. Drywall usually has an air gap on the cold side to let air circulate.

Drywall is very rare for non -internal walls across Europe, especially in cold countries like Denmark.

19c is basically a rule of thumb measurement.

Warm air can hold more moisture than cold air. Humans and their activities give off moisture. In winter humans maintain warmer than ambient temperatures. Therefore at some point the warm wet air is going to cool and reach its dew point.

Ideally you don't want this happening in the house, whether it does depends on the inside temp and the outside temp and humidity inside and out.

So there's a few knobs you can twiddle, temp being one.

https://sparenergi.dk/forbruger/spar-energi-i-dit-hus <- Danish Government Office for Energi (but in Danish).

They say 18 degrees, to avoid issue with moisture. It very much depends on how the home is built I'd assume, because holiday homes are normally kept colder, down to 5 degrees, during the winter.

Warning, annecdata: we turned down our heating signifcantly this winter, and both my roommate and me have gotten quite a bit of mold on and around our windows.

The thermostat is now back up to 19.5°C.

If you're able to invest in better windows (good double or even triple glazing), that will solve it. But just wiping down the window sill semi-regularly works well, even if it's an annoying chore.

With better windows the moisture may condense in other less visible places. Best to remove it with desiccator.

Sure, but there are insulation solutions that mostly prevent it. For example insulation foam layered between vapor permeable foil on the cold side and vapor inhibiting foil on the warm side.

But drywall (and other porous surfaces) are supposed to help regulate moisture, so if they never get an opportunity to dry because it's permanently cold and humid, yeah, that'll be a problem. Cracking a window at least 30 minutes per day (ideally on dry days) does wonders, though it won't help temperatures in winter.

Yep in theory it's all nice and easy, just put foils in and open the windows sometimes.

In practice... say, there is a dog that likes to gnaw on corners and thus punctures the vapor barrier. Happened to my own house's outside insulation. That's just one of many "unforeseen" ways how these foils and barriers stop working. I fully expect the polystyrene there to get moldy in 20 years or even less.