Have all avatars disappeared or just part of them? Custom avatars are basically Uploads, do other uploads work as expected?
Go to the rails console and check the record of non-functional avatar in database… Do they have correct URL, filesize, width, height, extension?
User.find_by_username('Overgrow').user_avatar
User.find_by_username('Overgrow').uploaded_avatar
They also need to have their optimized versions present… You can check that by:
OptimizedImage.where(upload_id: upload_id).where(version: 2)