I have tried again putting a roof (a really big one), but nothing changes.
ko with roof.plan
Reducing the intensity of the sunlight, only makes the problem less evident, but it is not the solution if I want a scene with a vivid sunlight (in the limit, for a zero sunlight intensity, the problem disapeers, but I don't have sunlight).
Could you please attach the "ko" plan file with the roof you put on it?