The installation of borgserver_ynh lead to this line added to authorized_keys:
command="borg serve --storage-quota 10G --restrict-to-repository /home/<user>/backup",no-pty,no-agent-forwarding,no-port-forwarding,no-X11-forwarding,no-user-rc ssh-ed25519 <KEYAAAAC3NzaC1lZDI1NTE5AAIIIIIIIIIIIIIIIIIIIIII1u8TD> root@<domain>
The installation of borgserver_ynh lead to this line added to authorized_keys:
command="borg serve --storage-quota 10G --restrict-to-repository /home/<user>/backup",no-pty,no-agent-forwarding,no-port-forwarding,no-X11-forwarding,no-user-rc ssh-ed25519 <KEYAAAAC3NzaC1lZDI1NTE5AAIIIIIIIIIIIIIIIIIIIIII1u8TD> root@<domain>
so the ssh connexion failed
to reproduce: just launched the installation on my stretch stable yunohost after added manually the gpg key as described here https://github.com/YunoHost-Apps/borg_ynh/issues/11