Comment 5 for bug 1572598

*facepalm*

Sorry, my fault.

The ssh-key was NOT a default "~/.ssh/id_rsa" but a "~/.ssh/id_rsa_backup"

Thus, the backup-script, that calls hat a line
"ssh-add /root/.ssh/id_rsa_backup" but it had no effect in a noninteractive env, where no ssh-agent was running.

Thus, there was not key for duplicity.

So i'm calling now duplicity with "--ssh-options" -

 /usr/bin/duplicity --ssh-options="-oProtocol=2 -oIdentityFile='/root/.ssh/id_rsa_backup'" ....

and everything works fine.