Investigate ssh-agent support
I've had a report that a no-password SSH yubikey setup does not work in Deja Dup, although it does work in Files.
From the user: "I have my SSH key on a YubiKey and have .ssh/authorized_keys configured on the remote server. I haven't configured a SSH password in Deja-Dup and my backups fail. If I connect beforehand to the remote server (using Remmina) and it's then mounted in my desktop session then a "Backup now" in Deja-Dup succeeds."
Using Files, they get prompted for a yubikey PIN entry. They're on the deja-dup flatpak on Fedora 34 Silverblue.
They use automatic backups. When it fails, they then try a manual backup, and get "Another Deja-Dup instance is running on this directory" -- which seems like a smaller related bug - they shouldn't be able to launch a new backup if the old one is still around.
I (mterry) have a yubikey, but it's ancient and only supports OTP. I'll need to get a newer one or figure out how to set up SSH with mine or set up ssh-agent in some similar way to emulate this case.