/3/ - 3DCG

Hello /3/
i've encountered a little problem while lighting my bathroom scene and i come here seeking your help. Made in 3dmax with mental ray.
the scene is lit with 2 free photometric lights placed behind frosted glass that is supposed to scatter it across the room. I've tried arch n design frosted glass and a physical sss material.
now for the question : how do i get rid of those white rectangle artifacts and the dark area on the corner ? what causes these ? how should i set up the glass material ? i've tried messing with fg settings. i've noticed the problems dissapear when i remove the glass from the lamps. that could be the easy way out but i want to learn how to properly make lights scattered by glass.