Comment 7 for bug 569418

I'm having this problem as well, and doing the gconf change described by Steffen solved it for me, but I think the G_IS_FILE assertion failure is a red herring -- the actual error seems to be on the duplicity command generated by deja-dup.

Below is the command executed when I had 'volume' as the value of /apps/deja-dup/file/type. As we can see, in this case duplicity exited with value 2 (i.e. error).
** (deja-dup:25730): DEBUG: DuplicityInstance.vala:199: Running the following duplicity (25779) command: duplicity collection-status --exclude=/home/salgado/.wine ...<elided-bunch-of--excludes>... --include=/home/salgado --exclude=/sys --exclude=/proc --exclude=/tmp --exclude=** --gio --no-encryption
** (deja-dup:25730): DEBUG: DuplicityInstance.vala:552: duplicity (25779) exited with value 2

And here's the duplicity command (which completes successfully) executed when I had 'normal' as the value of the aforementioned config.
 ** (deja-dup:25963): DEBUG: DuplicityInstance.vala:199: Running the following duplicity (25974) command: duplicity collection-status --exclude=/media/548710d6-fa6b-4348-8c5d-49b1718fbb0f/backup/gorducho --exclude=/home/salgado/.wine ... --exclude=/tmp --exclude=** --gio --no-encryption file:///media/548710d6-fa6b-4348-8c5d-49b1718fbb0f/backup/gorducho --verbosity=9 --gpg-options= --archive-dir=/home/salgado/.cache/deja-dup --log-file=/tmp/deja-dup-ZTZOBV
** (deja-dup:25963): DEBUG: DuplicityInstance.vala:552: duplicity (25974) exited with value 0

Actually, I've just realized that the duplicity command might be wrong because of the same problem that the caused the assertion to fail, given that I don't see the assertion failure when it manages to backup successfully.