There were artifacts on the render image like in the picture when I tried distributed bucket rendering. Horizontal lines along the edges of UVW MAP tile.
I decided to render the same scene on the slave machine on its own, but the same thing happened, so the problem is not DBR.
I double checked every setting I change during DBR and I figured out that it happens when "Use mental ray map manager" option is enabled. (I thought it s recommended for DBR so always enable it as a second nature)
My problem is solved however I'm curious: Is there any known reason for this?