Thank you for letting me know Carlos.
It is a strange one. I did hours of painstaking testing yesterday:
1. 8 Forest Pro objects with a single sided mesh as surface, stacked on top of each other to distribute office furniture throughout a building.
2. I removed the material from each FP object, and added them back, one by one, checking the scene loaded (xref) each time. I narrowed it down to 4 problem multi sub mats that cause the crash
3. I then removed each custom geometry item from one of the problem FP objects, and added them back, one by one, in order to identify 1 multi sub material that could be causing the issue (as FP consolidates all the mats into one auto material, with 100's of mat ID's)
And guess what? I added each geometry item back in, in the same order in the geometer roll-out, and the damn scene no longer crashes with that FP object!
I have no doubt this is an issue with 3dsMax, as let's face it, it always is. The fact it xrefs fine using the 3rd party plugin/maxscript SmartRefs, is quite damning.
I'll carry on with some testing and let you know of any significant findings.