Not sure what happened, but after upgrading to the latest beta, all custom user avatars are gone, replaced by a standard image of a light-gray user.
The default-generated avatars for users with no custom avatar uploads are not affected.
The image links to those avatars are:
https://xxxxx.com/user_avatar/xxx.com/user/50/179_2.png
EDIT: Uploading a new avatar image will show up. Therefore, all the previously-uploaded avatar images are all lost.