Mixamo Exporting Unknown Error

[janewhc]

I'm pretty new to animating so I don't exactly know much about it. But I've been trying to animate Roblox avatars and everything worked smoothly. I rigged everything properly and the animation works, but when I export, I get the message saying: "ERROR occured on export: Unknown error while generating motion" How do I fix this?

[Ussnorway]

the image shows a model breaking up due to weight map issues, I'd remake the character to have better range of movement i.e, more polly around the joins