Thanks for the detailed instructions. I could problem the problem here.
It happens because the objects stored in the "forest_templates" layer. These are used as templates for billboards.
When a new Forest object is created, the plugin checks if templates already exists in the layer to use them. If not, they are generated.
In this case, when main scene is loaded, templates already exists in XRef''d scene... but rotated because the dummy bind. Forest cannot handle them correctly, causing the geometry artifacts.
It can be fixed forcing to regenerate the templates in the main scene, not in the XRef'd. In this way:
1) Open the XRef scene.
2) Using the Layer Explorer, select all "_forest_template_" items in the "forest_templates" layer, and delete them.
3) Taking care to not edit any FP object (if not, templates would be recreated), save the scene.
4) Open the main scene, select any FP object and click "Update All" from the Display rollup.
Please note this problem doesn't happen with standard objects, only with billboards.
I hope that helps. I also reported the 404 error to our webmaster. He will fix it as soon as possible.