[Yafray] Strange light in the box's corners

See this image:


I’ve tried several settings, PhotonMap+Cache, only Cache, I’ve tried also to tweak the GI parameters a little but the problem is always there.
The light is coming from an area light (with energy set to 1) that’s below a plane of the same dimensions with emit enabled. In the GI settings, GI pwr is equal to 10.
Can you help me?

It appears as if light shines through the corners… This is a typical bug with Yafray, to avoid it, add another Cube around the room, so that no light can enter then scene from outside, I think that should work…