hi,
in this post, i show how to get rclone to calculate the md5 on a synbox.
https://forum.rclone.org/t/on-some-sftp-servers-e-g-synology-the-paths-are-different-for-ssh-and-sftp-so-the-hashes-cant-be-calculated-properly/36506
if i intentionally set --sftp-path-override to the wrong path, i get the same error as you.
not an expert with linux but
another possible issue, that you run sudo to get md5sum to work.
i would think user paperless, needs permission to access the files without root.