Commenting out that line breaks backup for me:
Attempt 1 failed. BackendException: File /tmp/duplicity-ZNKYnk-tempdir/mktemp-oRd6Y4-2 not found locally after get from backend
Attempt 2 failed. BackendException: File /tmp/duplicity-ZNKYnk-tempdir/mktemp-oRd6Y4-2 not found locally after get from backend
Attempt 3 failed. BackendException: File /tmp/duplicity-ZNKYnk-tempdir/mktemp-oRd6Y4-2 not found locally after get from backend
Attempt 4 failed. BackendException: File /tmp/duplicity-ZNKYnk-tempdir/mktemp-oRd6Y4-2 not found locally after get from backend