Preload doesn't work

Preloading of my posts has stopped working. When I click on the preload button in WP-Supercache I receive the notification email that preload has started but no posts are being supercached.

My debug file is here: http://www.datasafexl.com/wpdebug.txt

Any ideas why this might be happening?

https://wordpress.org/plugins/wp-super-cache/