Issues that i could spot and how to solve:
- Weightpaint issues: From my experience, sketchfab seems to calculate weightpaint a little different than blender. A similiar issue was mentioned in this thread once: Sketchfab is separating the edges of animated models?
Exporting to fbx gives usually better results with that. After a look on your model you also seem to have too many vertex groups affecting the model. I deleted every vertex group except for the “DEF- groups/ Bone.001…005” and it worked fine. Those are the ones we need afterall. However they do need a little manual fixing, so you should check on that part(jawline for instance). Perhaps sketchfab got confused with this many many vertex groups or something? don’t know…
- Kneejoint rotation: I experienced something similiar with one of my models, and I’m not sure why, but perhaps there are issues with how sketchfab handles bone-constraints? I got that fixed by simply baking the animation. Search with f3, select bake action. Framestep should be between 5-10( otherwise way to many keyframes). Enabling all checkmarks will ensure animation will play and get exported correctly. Or maybe increase distance of the poletarget bone.
EDIT: Why do you have 3 Armature modifiers on your mesh?