Light halo around windows with Global Illumination

Hi, I finally came to start working on lights in my scene. I enabled Global Illumination, but I have small problem with windows where light coming from (one light source - directional light).

There is some kind of halo (light bleed) around windows. Wall is properly thick so it can’t be good old light bleed from thin walls.

Does anyone know what causing this?
Thanks for help.

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