Prefabricated house is leaking. We feel cheated

If a window is removed, the fault or defects should be identifiable. Whether it is actually due to the windowsill is only a supposition. If serious faults are found here, the rest would become clear by itself.

And if it is a simple aluminum bench, you can remove it minimally invasively without extensive destruction. Ideally, you obtain a sack / bucket of the plaster currently on it beforehand. After 8 years, most of it should still be there. Then you can seal the plaster surface again immediately after the repair.

A photo might possibly also help to provide some clarification. The approach of @ Domski is probably the simplest and cheapest solution.

But after 8 years? Well, that does mean you need a really long breath; especially because it has to be fraudulent or particularly fraudulent; and proving that?

So I would have an expert take a look at it; and then in the next step have 1-2 windows treated accordingly by a local craftsman

And only if the local craftsman is willing to take it on! That’s also a thing.

Short update. I spoke with a local window installer. He wants me to send pictures and then he will decide if he can do anything about the window, or he will come by and take a closer look. Apparently, every prefab house manufacturer has a slightly different system for sealing the windows. Attached is a picture of the leaking spot on the kitchen window. It looks similar on the opposite side.

Are you basing your question solely on these small spots and the measurement by the site manager? I am also missing an overview shot to see the location of the spot on the building. At the risk of making a fool of myself: I don't see anything here that would worry me. After 8 years, the plaster is no longer like new.