I'm new to Mixamo, because I'm taking a 3d class for the summer, and I keep running into problems. My main issue at the moment is when I import the character model I made in Blender to Mixamo for the auto-rigging. It's always after I upload the character that parts of her head go missing.
She has hair strands on the sides of her face, so after I upload, the outer faces of them disappear, leaving them hollowed out, if that's the right term. And I can't fill in the vertices after moving it to 3ds Max
So she isn't hollow in Blender then?
Blender exports as Wavefront Obj ... Then open that exported model in whatever 3d viewer you care to use and confirm the model looks correct before moving on to Mixamo is the work flow I would recommend at this point so we don't have to pull any hair out on a problem that may not even be at the Mixamo end