Fixed glazing plastic window is leaking

Sure?

Pretty sure. But no chance I'll get that on the photo. However, if I shine light in, I see the outer metal profile of the frame, the corresponding hole, and how mounting foam is sticking out of that hole...

Not just one, but all screws are missing in that window. Which led me to find a second window in the house where this is the case. There too, all the screw holes are empty.

How many windows with fixed glazing are there in/at/above/below/... your house?
My crystal ball believes there are exactly 2 pieces because such windows are often mounted with anchors/brackets/... that are invisible after the jamb is finished. Did you take any photos during the construction period where something might be visible?
Do the windows come from the developer/... or from a window manufacturer or ...?

I have five windows that are like this, and two have no visible screws. The smaller window in the pictures is the second one. There are anchors, you can see them in the pictures. It would not hold with just mounting foam. The house was built with Heinz von Heiden, they always choose different companies for each trade. I need to find out who the window fitter was in the end.

Solid house with [WDVS]? Did I understand that correctly.

In front of the window on the upper floor there is nothing, no balcony/grate/glass pane/...? Was there a construction supervision commissioned by you?

All these windows are on the upper floor and all without balconies. That is why the fixed glazing. No, I did not have a construction supervisor.