Like I have seen mentioned here as well as elsewhere, I had the issue with Texgenx64 failing to execute correctly on the /meshes/landscape/grass/ivydrj.nif from the Cathedral Grass Mesh Library mod that is used by Freak's Floral Fields, among other grass mods. It would basically always fail to render the billboard, giving an OpenGL Invalid Operation Error, but I did end up pinning down what exactly was causing that particular error at least in my case by another Grass Mesh that had gotten updated/changed since my last run and was always failing 100% of the time. I have been using PBR textures in my mod list, which needs PGPatcher to patch nifs, and it turned out that from having the mod https://www.nexusmods.com/skyrimspecialedition/mods/149315 installed, it would patch /meshes/landscape/ferngrass.nif to add the environmental mask that was added in that mod, which would get the exact same error that I encountered in ivydrj.nif, and by comparing the old and new mesh, it appears that texgen, at least in certain driver setups, will fail to render nifs that are set to the environmental mask shader type. If you go into the nif in nifskope and change the shader type to default, it is able to be rendered in the HD Grass preview just fine, as well as at least across numerous runs, always be able to set up as a grass billboard. I'm not sure if this is a constant or just setup/driver based, but at the very least that was the common demoninator that I found, and by changing the shader type it would complete successfully many times. Just thought I would report that particular finding, for anyone else who has been having that error with that particular nif, or perhaps even others, and hopefully it's able to be fixed on the texgen side as well.