Tried uploading an avatar with the latest SDK version - it'd fail at the last second, claiming there's no thumbnail when there IS one set for sure (tried with multiple kinds + in-scene capture). But the avatar could be seen as uploaded on the VRC website, but not in game... and when the 'change to avatar' button was clicked, it'd claim there's no avatar there.

I rolled my SDK back to 3.8.2 and it uploaded just fine.