Thanks for this thread – saved me a bundle of time figuring out what was wrong with the backup plugin.
Question though, does the HEAD Request filter need to be removed for any plugin that uses wp-cron.php?
Not necessarily and not usually. Typically the problem that can occur with Crons, such as scheduled backup crons is that the wget command is being used instead of the lynx command. BPS blocks the wget command. So you can either remove the wget command from BPS security filters or use lynx -source instead of wget -O. http://forum.ait-pro.com/forums/topic/wget-gives-403-error-when-bps-is-active/