strange mozaic when using vray2sided material

[lukx]

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.

[leo.surrealismo]

are you using solidRocks?

[lukx]

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

[leo.surrealismo]

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