On fbx import, my avatar setup hangs at 99% and on ‘Uploading Results’ with this error that may be related (it prints every time I try to use the autosetup). Waited about 40 minutes on this bar for nothing to happen, no existing resources I can find on this issue. I’ve got all my FACS expression set up, cage is watertight, HumanoidRootNode exists in bone structure ect, just want some ideas as where to look next. 
Having an issue with this as well, if anyone can help please do.
Replying to this: I did fix this.
When this happens it’s basically an unknown error with the avatar. Yes, I know, very unhelpful, but you’re gonna have to fix the modelling on it in some sort and you should probably get either another error or an avatar fix.
For me this was happening because the torso looked weird.
For me, this was ‘fixed’ by opening a fresh studio project, so nothing to do with the export at all. Super frustrating, but glad you found your own fix