Déjà Dup over SSH - where to enter my login credentials?
Déjà Dup uses GVFS to access the server, which is the same system that is used when you log into an SSH server with Nautilus’ “Connect to server”. You could try logging into your server using Nautilus and see if it works then.
If you use key-based authentication, you should be asked for your password as soon as Déjà Dup tries to connect to your server.