Error on locked files causing deja-dup "Backup Failed"

Deja-dup backups display a pop-up window (likely since 0.7-r1224, 0.8-r1111, 2016-06-13) saying:

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BACKUP FAILED

Error accessing possibly locked file /etc/.pwd.lock

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This is likely an error in duplicity 0.7.08

Running:
export DEJA_DUP_DEBUG=1
deja-dup --backup

it looks as though the backup is actually completing successfully, but is displaying an error screen. The duplicity command itself can be obtained from the output of the above command:

/usr/bin/duplicity --exclude=/tmp/1604_dup --include=/home/aaron/.cache/deja-dup/metadata --include=/etc --exclude=/home/aaron/Downloads --exclude=/home/aaron/.local/share/Trash --exclude=/sys --exclude=/run --exclude=/proc --exclude=/var/tmp --exclude=/tmp --exclude=/home/aaron/.xsession-errors --exclude=/home/aaron/.steam/root --exclude=/home/aaron/.Private --exclude=/home/aaron/.adobe/Flash_Player/AssetCache --exclude=/home/aaron/.cache/deja-dup --exclude=/home/aaron/.cache --exclude=/home/aaron/.local/share/Steam --exclude=/home/.ecryptfs/aaron/.Private --exclude=** --gio --dry-run --volsize=50 / file:///tmp/1604_dup --verbosity=9 --gpg-options=--no-use-agent --archive-dir=/home/aaron/.cache/deja-dup --tempdir=/tmp

Running this command directly, duplicity appears to run correctly, so this is likely an issue with the duplicity error changing slightly and triggering the "Backup Failed" warning.