Humidity permanently too high, mold formation, cause?

Or does it simply accumulate there because it is in the corner of two exterior walls and at the very bottom, thus probably the lowest point and the coldest, and also without air circulation due to the nightstand?

Exactly that. The corner cools down the most because it loses heat from two sides.

So the wall itself is fine, moisture from the inside.

How should I proceed best now to get the wall dry? The corner is also the hardest to heat, even with the heater on. Put a fan heater in front and have it blow air?

In the future, place the nightstand differently so that there are 10cm of space all around, that's clear. But first, get the wall dry again.