Thanks for confirming! After seeing this I re-adjusted DISCOURSE_S3_CDN_URL: back to amazonassigned.cloudfront.net hoping it my clear the cert and url issues.
The rebuilt and re ran all the rakes mentioned in docs.
rake posts:rebake
rake uploads:migrate_to_s3 still generates FileStore::ToS3MigrationError
rake posts:rebake_uncooked_posts
rake s3:upload_assets
rake s3:expire_missing_assets
Still no joy on getting the site to load.
Any suggestions?