If an avatar contains an EditorOnly-tagged object which contains a child sub-armature (sub-animator?), the upload fails. This works with the current live SDK version (and unity 2017), and fails with the beta SDK and unity 2018.
This particular issue was discovered when integrating Tsubokura-san's haptics suit assets with my avatar, and then making the debug components invisible on upload by marking their root EditorOnly.
Minimal repro prefab: https://www.dropbox.com/s/nwhmmoj98pjo6v2/bug%20repro.prefab?dl=0
If you drop this in your scene and click build&publish, it will be cloned, but the build will stop there. If you clear the EditorOnly tag on Sub-armature, the upload will proceed.