As soon as I use Forest Pack together Civil View elements, Mental Ray Max crashes, what is the problem ?
In order to render the high-poly trees efficiently, Forest automatically sets up the following options in Mental Ray:
But it seems the Civil View elements are not compatible with "Use Placeholder Object" mode, crashing Mental Ray.
So, if you don't use high-poly trees in your scene (for example using Forest billboards), you can turn off this option. If not, the only way is to exclude the Civil Elements from the render.