Mopper.exe is the collision optimization module. The _far.nif files in LtPD do not have collision (as expected) so that's not what is crashing the setup. As I said, there's one mesh in the unmodified LtPD that DOES crash mopper.exe because of something horribly broken in the collision on it, but I don't think it was a major outdoor aqueduct piece.

I've also never had an issue with using the clean_farnif spell on it.

Oh, okay. It didn't cause a crash for me with older versions of PyFFI. I thought it was a _far.nif file because it crashed when I generated LOD. I'll just go without optimizing those meshes.