I’ve had this. It seems that some of your ‘cores’ are running out of memory. In my case I reduced the size of the buckets (the ‘region division’ in ‘system’ settings).
I was using 128 and I reduced it to 64 and it fixed the problem. (I am assuming you are using a multicore machine).
I had those issues in the past on busy areas in renders, around leaves of trees or tight repeating pattern areas.
In past versions of Vray, I would make sure to hit the screen size ratio button on the output flyout section. So choose your output size and then hit the screen ration button. That seemed to stop that from happening most of the time.
In the most recent version, I tried changing the region size as dcauldwell suggests. That did work as well, maybe more consistently, even when not setting output to the screen’s ratio.