I have spent a few hours trying to figure out why I cannot export one of my characters to FBX in CC3, while I had no problem exporting it in CC2.
Finally I have found a cloth name which had a white space at the end.

To replicate:

1. Within the default project add a white space at the end of any name.

Like rename "Bra" to "Bra "

2. Try to export to FBX

Suggestion: Evaluate names and Trim white spaces. Users might accidentally add some and then would have a headache trying to figure out the problem with export.