I got the below error when restoring from backup after upgrading from v0.30 to v0.40.
CollectionsError: No backup chains found
It was caused by the below line. I failed to add the directory encrypted after backup
sudo -E duplicity restore --force file:///tmp/home/user-data/backup /home/user-data/
I made the below correction and the restore worked.
sudo -E duplicity restore --force file:///tmp/home/user-data/backup/encrypted /home/user-data/
I hope this helps someone.