How to avoid strange blocks?

[hill_lam]

How to avoid strange blocks appearing on the ceiling?
I placed a opacity PNG image outside the window to create the tree shadow, which cause those strange blocks.
How can I solve it? I'm also very interested to know the reson and principle behind it.
Thank you!!

[hermit.crab]

Try disabling the "Adaptive Dome" option in the VRayDome and see if that makes any difference. In any case, do send the scene so we can debug it.