So #Mastodon v3.1.4 now stores remote media in a new "cache" directory. It was automatically created on my S3 storage provider (my own, running #minio) during the upgrade, but wasn't assigned an access policy, so all new media files turned out unreadable.

Had to do a "mc policy set download media/mastodon/cache" to make it accessible to the public.

#mastoadmin