While testing MDL-29670 I noted that automated backups assumes a users context exists.
In my case I had several users uploaded via CSS that have not had contexts created for them yet (never logged in or operated on other than enrolment).
This led to the following notices popping up:

Notice: Trying to get property of non-object in /private/var/www/integration/backup/moodle2/backup_stepslib.php on line 1771

That line of the file is:

$userctxid = get_context_instance(CONTEXT_USER, $userid)->id;

and a quick check of the database confirms that several users in the course do not have contexts yet and that upon logging in as them the issue is resolved.

Cheers
Sam