I think the following error must arise if backupping to a proftpd target via SSH,
because proftpd does not provide a shell but is SFTP-only.
Or am I mistaken?
I ask if you plan to implement SFTP connections too?
Uploading archive to SSH server SERVER ..
8 SERVER backed up successfully, 0 had errors. 4 Virtualmin configuration settings backed up successfully.
Deleting backups from /db_TST/%Y-%m-%d__%H-%M__string on SSH server SERVER older than 30 days ..
.. failed to list directory via SSH : USER@SERVER's password: exec request failed on channel 0
Disclaimer: I am GPLian