I have similar problem. When I have Forest applied in some complex scene, after rendering max doesnt crash, but not responding for a minute or so and after that its ok. I think its caused by Forest and VFB (VFB is really slow, when this happen). Its happening only in some scenes in max 2014 and 2015 with newest forest pack. Now Iam going to install beta version in max 2016, so will see in there.