Thanks for reply!
I made a new clay rendering studio and render time is "a little bit" higher, but finally I mnaged to get rid of those shadows. I think that they were caused by wrong lights in the studio.
Previously I had a modelled shape with a vray light material assigned to it, so render times were faster but quality was worse.
I have also tried LightCache but I wasn't satisfied with the results so I moved back to Quasi-Monte Carlo.
I hope that render looks better now I'll still experiment with colours and intensity of those lights.