The problem was in the template objects used by Forest to create the billboards. By default, these objects are in a hidden and freezed layer, but by some reason they were modified in your scene.
How to fix it:
- Open the Layer Manager, and disable "Hide" and "Freeze" checkboxes of the layer "forest_templates"
- Select all "_forest_template" objects from the scene (use "Select by Name" tool)
- Delete the selected objects
- Save the scene, reset and reload
- Select any Forest object and modify some parameter to force a rebuild