As far as I am see the borg app lacks a descritpion how to set up the service correctly.
A new ssh key was generated under /root/.ssh/id_borg_ed25519
But one should inform the admin to put this /root/.ssh/id_borg_ed25519.pub
at the end of the authorized_key file on the borg archive server.
one could test at the commandline on the yunohst like this
sftp -P 23 -i .ssh/id_borg_ed25519 uabc@uabc.your-storagebox.de:yuno
It should work without password