Downloaded Freeside Open, after checking the archive I already knew what's wrong even without installing it. There is a new mesh for fort meshes\rzw\freesideopen\oldmormonfort2.nif and it requires the corresponding _lod.nif mesh for LOD which is missing. So you either need to create a new lod mesh for it, or get the vanilla one
meshes\architecture\strip\oldmormonfort_lod.nif
and copy it as
meshes\rzw\freesideopen\oldmormonfort2_lod.nif
However vanilla lod mesh contains opened gates which were removed in rzw's mesh, so there will be gates in lod but no gates when up close.
I think this was the last reported issue for New Vegas. Time to move on to TTW FO3 worldspaces and DLCs and hunt for bugs there. So far the common issues are:
1) Black rectangles - no alpha in diffuse LOD texture
2) "Deformed" meshes - translation/rotation set in lod mesh itself
3) Missing meshes - lod meshes are actually missing, could also be the above case with large translation value that moves lod out of sight (under the ground for example)
4) "wrong" texture in lod, well I guess it is just a very low quality of lod textures in general
Anything else?