Dor․
The error "You must provide a path to the thumbnail image when creating a new avatar" which is intended for new avatar uploads blocks updates to existing blueprints if the build is started before the thumbnail is (down)loaded.
Repro:
  1. Have an existing avatar project, with the SDK panel hidden (closed or docked but tabbed away)
  2. Open a new session of Unity for the project (thumbnail not cached)
  3. Open/switch to the SDK panel and immediately hit "Build"
In theory between 2 and 3 you'd have made changes to the avatar and then decided to upload, which is exactly when I encountered this, after thorough testing in Unity I decided to upload to test in VR, but it was the first time I tabbed into the SDK this session.
Likely affects most/all SDK versions with the newer build panel.