Timber frame construction - wet floor due to airtight layers?

Sure. Attached is a photo. But is it particularly meaningful? I guess there were about 5-10 liters of water distributed over approximately 25 sqm of floor area.

Photo unfortunately doesn't say much. I believe it's condensate caused by the double foil. Let it dry and only cover with foil on the top, let it "breathe" at the bottom. It's very difficult with the little information... Will you tell me the [Bundesland]?

In my experience, KVH does not dry out in an unheated envelope but rather tends to absorb moisture. However, if you can really rule out water ingress from the outside, then there is no other possibility.

My calculation out of the blue from #4 is super theoretical and, of course, only works if the environment of the wood is correspondingly super dry and at the same time not capable of absorbing the water in the air. This is super theoretical and a great excuse. The water definitely does not come from the wood. If it did, I would immediately adopt this method for drying firewood. Don’t let yourself be fobbed off.

It is completely impossible that the KVH emits that much water.

The water penetrating from the outside can be ruled out.

Often the simplest explanation is the best, even if it is generally excluded.