Mold inside a 3-year-old house wall/ceiling corner

Most likely, there is no construction defect.

It is in accordance with the state of the art that the planner/architect creates a ventilation concept according to DIN 1946-6. If no one has pointed this out to you or it was not done, a construction defect can very well be derived from this.

You mean they should have told him that he has to ventilate 5 times a day? And because they didn't do that, it's a CONSTRUCTION DEFECT? No way.

It has to be repainted now anyway because of the stains, so while you're at it, insulate the 2 exterior walls beforehand with gypsum board styropor composite panels, plaster them, and then paint. The additional insulation reliably solves the problem once and for all.

insulate the 2 exterior walls with gypsum board-styrofoam composite panels,

a: wrong material
b: the entire wall? Why?

It is possibly (most likely even) a heating/ventilation problem. I consider it premature to bring out the insulation wedge already.

And if so, (also most likely here) insulation wedges would suffice.