Perhaps uninstalling it and reinstalling it would fix it?
sudo apt-get update
sudo apt-get remove deja-dup
sudo apt-get install deja-dup
I believe deja-dup uses
duplicity under the hood, so perhaps you'd still be able to restore archived files using the command line in the meantime?
You can backup easily using duplicity at the command line to make a backup:
duplicity --no-encryption --verbosity 8 /home/user/folder/to/backup/ file:///home/user/destination/to/store/backup/
The first time you run this command, it will do a full backup, and each time there after it will notice that there is already a full back up at that destination and do an archived backup instead.
I have cron job script that runs a command like this, except I've got it to where it always backs up to a folder named after the current month and year. This causes it to do a full backup the first day of every month and then archive for the rest of that month, until the destination folder changes to the current month (next month).