Thanks fo the scene. We've identified the problem:
When Forest uses a surface as scattering area (no area splines), it extracts a "virtual" spline from the closed edges of the mesh, and it's used as spline area.
Depending of the surface, this spline may more or less adequate to be used as area. In this particular case, it seems the resulting spline has several self-intersections, that creates an endless loop in one of the Max functions wich process it.
How to fix it, extracting the spline manually:
1.- Create a copy of the surface object. By default, Max collapses it to Editable Mesh.
2.- Convert the object to Editable Poly.
3.- Set "Border" edition mode.
4.- Select all (Ctrl A).
5.- Edit Borders->"Create Shape From Selections". In the options dialog, select "Shape type" as Linear.
6.- Select the Forest object.
7.- Create a new area spline, assigning the shape extracted above.
8.- Select the "Surface area" and turn it Off, so Forest doesn't generate items from it.
Now you can set Size/Edge mode in the area spline safely.