The site is on the same VPS as several others, all of which backed up without issue. This site is (of course) the largest of the set, with an active Kunena forum, and large JoomGallery photo gallery.
Issue was happening on Akeeba Backup 3.11.1, I was attempting to back up before upgrading to 3.11.2 and Joomla 2.5.21. :-) Upgraded to Akeeba Backup 3.11.2 to try an resolve the error, but no luck.
Occasionally, it gets past backing up the DB, but then gives a 500 error while backing up the files.
Log is attached. Additional info can be viewed at: http://www.knightrealms.com/phpinfo.php
Thank you for your help!