See: https://moodle.org/mod/forum/discuss.php?d=319114

I have had multiple automated course backups fail over the last week. Some successfully complete but others fail in the same minute that the delete stale temp files task kicks off. The automated backups task overlaps with the delete stale temp files task. Here is the cron output snippet from the most recent failure during that cron run.

Server Time: Thu, 03 Sep 2015 00:57:01 -0400

Execute scheduled task: Delete stale temp files
... started 00:57:02. Current memory use 18.4MB.
backup_auto_failed_on_course DemonChem
Exception: directory_not_exists /var/www/vhosts/rjreynolds.org/moodledata/temp/backup/25d3b4b9248732c758fca0aa6588b48c/activities/quiz_7194
complete - next execution: Fri, 04 Sep 2015 00:30:00 -0400
Backing up e-Commerce I (Blair)...
... used 0 dbqueries
... used 2.2312309741974 seconds
Scheduled task complete: Delete stale temp files
Execute scheduled task: Cleanup files in trash
... started 00:57:04. Current memory use 19.5MB.
Deleting old draft files... ... started 00:57:04. Current memory use 21.8MB.
done.
Deleting orphaned preview files... ... started 00:57:06. Current memory use 21.9MB.
done.
... used 11 dbqueries
... used 2.2669699192047 seconds
Scheduled task complete: Cleanup files in trash