Concern about vapor barrier - air from ceiling outlet

To rule out leaks in the air- and vapor-tight layer, the only thing that comes to mind spontaneously is measuring the surface temperatures at the questionable spots. Maybe someone else has better ideas.

If the draft came "from the inside," that would be my preference. Against this speaks the fact that it is actually only noticeable in stronger wind (outside) - and then also "noticeably cool" - so rather not room temperature...

Cavity behind the drywall

Is it possibly not sealed towards the attic? Vapor barrier pulled up to the collar beam layer and then further below the ceiling, and now cold air from the attic is whistling through the installation space?!

Edit: Sketch added. Red = vapor barrier, green = ventilation

... could be. Maybe I can get to it first because there are screwed chipboards on top. On the other hand, the draft on the slope on the weather side is significantly stronger....

That would not contradict my assumption. On the weather side, you also have the wind, and if it blows through the attic between the tiles, you probably have more drafts on this side in an open installation space than on the lee side of the roof.

Thanks already... but I see it correctly: no matter where the connection between the room air and "behind the vapor barrier" is - mold is threatening? Or can one trust that the wind blowing behind it will always dry it out again?

(I am a bit panicked about mold because in our last rental apartment the builder had "forgotten" the insulation in the upper flat roof - and during the first -20°C winter the underlying edges turned black within a very short time...)

I just realize that I wrote/drew nonsense. According to your description, the vapor barrier is on the insulation, not on the installation layer (which would have been a strange construction). Sorry!
So it probably comes down to the issue described by . Somewhere there was sloppy work, and now the spot(s) need to be found afterwards.
In my opinion, a professional should handle it, who also has the necessary equipment to carry it out minimally invasively (e.g. endoscope).

Untreated, this does not necessarily have to lead to mold in the end, but it can.