Getting the same error due to 3 posts 
Updating the URLs in the database...
Warning: no type cast defined for type "name" with oid 19. Please cast this type explicitly to TEXT to be safe for future changes.
Removing old optimized images...
Flagging all posts containing lightboxes for rebake...
123456 posts were flagged for a rebake
rake aborted!
FileStore::ToS3MigrationError: 3 posts are not remapped to new S3 upload URL. S3 migration failed for db 'default'.
/var/www/discourse/lib/file_store/to_s3_migration.rb:131:in `raise_or_log'
/var/www/discourse/lib/file_store/to_s3_migration.rb:86:in `migration_successful?'
/var/www/discourse/lib/file_store/to_s3_migration.rb:357:in `migrate_to_s3'
/var/www/discourse/lib/file_store/to_s3_migration.rb:65:in `migrate'
/var/www/discourse/lib/tasks/uploads.rake:245:in `migrate_to_s3'
/var/www/discourse/lib/tasks/uploads.rake:224:in `block in migrate_to_s3_all_sites'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/rails_multisite-2.5.0/lib/rails_multisite/connection_management.rb:76:in `with_connection'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/rails_multisite-2.5.0/lib/rails_multisite/connection_management.rb:86:in `each_connection'
/var/www/discourse/lib/tasks/uploads.rake:222:in `migrate_to_s3_all_sites'
/var/www/discourse/lib/tasks/uploads.rake:218:in `block in <main>'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `<main>'
Tasks: TOP => uploads:migrate_to_s3
(See full trace by running task with --trace)
@gerhard - If we get an error like this, do you know if the meaning is “success! - except for 3 posts” or “major migration failure, because of 3 posts”?
To attempt to resume, should I run rake posts:rebake, rake posts:rebake_uncooked_posts or rake uploads:migrate_to_s3 again?
Would appreciate any help 