The user already exists or is he being created as part of the SSO log in?
Also is /sidekiq with a queue? Any errors regarding DownloadAvatarFromUrl?
Also try this in a rails console in a production instance:
user = User.find_by_email(AUSEREMAILHERE)
Jobs.enqueue(:download_avatar_from_url, url: 'http://mycoolavatar.png', user_id: user.id)
and watch logs.