I fixed it. I'm going to put the fix here, so everyone else might be able to use it as a guide if they have the same issue as I do. Essentially, first thing to do is disable your antivirus when doing this (could also try just adding an exclusion/exception rule). Then what I did was that I first disabled all mods in MO2, then I slowly enabled them one at a time. Everytime I enabled a mod I ran FNIS to check whether or not it was causing the error. What I learned from this was that the only fix to this issue is to simply delete ALL of the mods that FNIS might've tried interacting or reading. If you don't, then you'll still get the error. ALL MODS that have anything to do with animations NEEDS to be deleted and re-downloaded, including FNIS itself. Even skeleton mods like XPMSE (I think that's what it's called) needs to be deleted and downloaded again. If you have a mod that manages animations, then it needs to be deleted and downloaded again. You also need to get rid of all the FNIS files in the overwrite. (Or just move overwrite, then have FNIS write files, then move overwrite back and say no to all.)