strange mozaic when using vray2sided material

I often use 2sided vray mat to soften the light in setup like this:

and I got this strange mozaic

I bumped all possible settings and can’t get rid of it.

are you using solidRocks?

no I don’t , not for this scene. I tried all gi methods and it shows up on all.

that happened to me before… I’ve resolved it when turning on retrace treshold.