Comment 37 for bug 1715582

Did you try all the settings that I described in this thread?

1. I had to create a /var/cache/samba directory (which you might probably have already).
2. The settings dialog has changed.

Storage location
  Storage location: Network Server
  Server Address: smb://server/share_name
  Folder: /complete/path/to/backup/directory

I had to play around for a while myself to figure this out. Setting the / was essential (as the path is concatenated as python strings).

On the backend, you must have write permission (the whole path, not just the destination directory).

I hope this helps.