In this case is not clear if the problem is caused by Forest, Vray or Onyx. And unfortunately we have not a copy of Onyx to reproduce the problem here.
I suggest you try to isolate the problem, removing progresivelly objects from the scenes (trees, grass, etc.). Once you know what object is causing the crash, convert Forest to stand-alone objects using Forest Tools. If it also fails, the problem would be in Vray or Onyx. If not, we will decide the next step.