Found it, for anyone having the same issue, the thermostat valve is broken. Located in the insulation behind the back panel, which comes apart when removing 2 screws at the bottom and one in the middle behind a small cover. Simple plug and play exchange and everything works fine again
I had the same issue with my Renault Espace once. Turned out that a simple small bracket was broken that held the windowpane in place. When I repaired it, I had to guide a wiretapped metal flexible rod through a piped area in the window mechanism far enough, otherwise it would have stayed in the down position as well. I did a yt search and found this, but my guess is that you already saw this... https://youtu.be/sT-cgqgzogw