Backups over SSH are failing to delete with directory error

Hi all,
Not sure which info you need here, just going to give it all…

Crux of it is that backups are writing correctly, but failing to remove old files. The error it throws is:
Deleting backups from /storage/backup/daily/%d-%m-%Y on SSH server local-backup older than 7 days …
… failed to list directory via SSH : usage: ssh [-1246AaCfGgKkMNnqs… snip]

I have 3 scheduled backups (daily, weekly, monthly) running. My destination lives on a dynamic IP. The destination SSH’s in (using cert authentication) and writes its current IP into the server. The server runs a script before backup which writes this IP to the hosts file, and a script which removes it after. The server uses cert authentication back into the destination. No passwords involved, all over SSH.

Backup settings are as follows:
SSH server: local-server:8080
User: backup-user
Pass: (cert-auth, so it works)
File on server: /storage/backup/daily/%d-%m-%Y
Delete after 7 days
Do strftime-style
One file per server
Create destination directory

Running system is:
Operating system Ubuntu Linux 16.04.1
Webmin version 1.860
Usermin version 1.720
Virtualmin version 6.01-3