Ugly Splotch on my Butt (see video)

[Vyxenne]

On several of my Nif Optimizer conversions and/or Outfit Studio projects/conversions, building the item in BodySlide results in an odd, blotchy black mass showing through the minidress from the front, and a dark smear or spot appearing on my butt when viewed from the back. Please see the video.

If it were only happening on one or two of the meshes, I would chalk it up to "bad Oblivion conversion" or whatever, but it is happening too often and on too many meshes from too many reputable sources (e.g. DeserterX). It is not affected by toggling my ENB off and on, or by any other setting I can find. In every case, the mesh and textures look completely normal in NifSkope and in Outfit Studio- only in-game do the ugly shapes show up, rendering the item unusable.

 

I would very much appreciate any help tracking this issue down. Thanks.

[Vyxenne]

Nobody has any idea why this is happening? It only happens when I convert a mesh from Oldrim to SSE via Nif Optimizer or OS+BS. Please, I really need help understanding (and fixing) this.