It's possible sudo is printing something to stderr, can you check that? My bet would be on a hostname name resolution (i.e., a warning).
** Changed in: rsync (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Server, which is subscribed to rsync in Ubuntu. https://bugs.launchpad.net/bugs/1830121 Title: rsync --rsync-path="sudo rsync" over ssh via pki fails due to protocol mismatch To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu-mate/+bug/1830121/+subscriptions -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs