Wet spot in the attic, what could it be?

Hello everyone

We are approaching the end of the construction phase. Yesterday, when the tiler wanted to start tiling the bathroom in the attic, I noticed the wet spot in the corner as shown in the picture. According to the architect, it could be condensation. The roofer is coming tomorrow to take another look at the roof, but he said it would be much worse if the roof were leaking.

Construction start 01.05.2017
Screed 9 weeks
Interior plaster I don't remember exactly but longer than the screed

The foil that can be seen appears to be wet/damp in the upper area (directly below the edge of the green drywall panels) at the connection to the wall towards the roof truss. From my point of view, this would argue against condensation water and indicate a problem with the roof. It looks as if the foil is "sticking" to the beam because of water. Or am I seeing this incorrectly?

Do you have pictures of the shell construction of the location? It could be a thermal bridge, planning error, or faulty execution and then condensate. Just measure the temperature on the surface. A pitched roof actually cannot be leaky.

I am surprised that you can simply see the edge of the foil at the lower part. It will be the same at the top, but you cannot see it because of the blur. Shouldn't it be glued somewhere? Is that the outer wall?

Faulty vapor barrier. Perhaps wall connections were not executed correctly. Surely the insulation is wet. Definitely check carefully, otherwise you will soon have a mold problem (and many others).

I should check the laptop for pictures of the shell construction tonight. We have a [Rheinmittelhaus]. It has rained heavily and snowed these past days; before that, there was nothing, which I would have noticed. What would you suggest I can do?