I am zeroing in on it, but it's slow going.
I can pinpoint the problem (for now, in this one scene at least) to one of my forest objects and PSD Manager. If I turn of either one it works, but if both are active it creates this problem.
I am going to merge to a new scene to see if I can narrow it down further, but one problem I noticed: when you delete a forest object it still leaves all it's template versions in the max file. As I had many large meshes the file is bloated as a result, and because the FPP library assigns a random name it's very hard to figure out which is which to manually delete them.
Can you make it so that the meshes get deleted when the FP object is removed, or if different geometry is assigned to it?