Hallo
Question
Are you using the backup software that comes with Ubuntu-Mate, “dejadup”, to make your backups?
see
https://wiki.gnome.org/Apps/DejaDup/HowItWorks
If you want a non-encrypted backup you would have to use another tool (as well as dejadup perhaps?). I use Grsync.
see
http://www.opbyte.it/grsync/
also useful
Does that help? 