The issue is that your wp-cron.php is not returning a 200 response when hit with a HTTP request originating from your own server, it could be several things, in most cases, it’s an issue with the server / site. There are some things you can test to confirm this is the issue.

• Are scheduled posts working? (They use wp-cron as well ).
• Are you hosted on Heart Internet? (wp-cron may not be supported by Heart Internet, see below for work-around).
• If you click manual backup does it work?
• Try adding ‘define( ‘ALTERNATE_WP_CRON’, true );’ to your `wp-config.php`, do automatic backups work?
• Is your site private (I.E. is it behind some kind of authentication, maintenance plugin, .htaccess) if so wp-cron won’t work until you remove it, if you are and you temporarily remove the authentication, do backups start working?

NOTICE: The plugin uses cron job system (WP Cron) to execute scheduled backup. For WordPress to recognize when to execute a backup job, your website (inner clock) needs to be set regularly. And this set happens whenever someone (including you) visits your site. If your site happens to not being visited for a period of time, WordPress’s inner clock gets sort of slow. In that case it takes an extra server-side cron job to regularly call http://yourdomain/wp-cron.php and tell WordPress what time it is.

A simple way to find out whether WP Cron works as it should on your site, try to create a new post and delay its publishing time by 10 minutes or so. Then leave your site (that’s important), come back after 11 minutes and check whether your scheduled post has been published. If not, you’re very likely to have an issue with WP Cron.

By default PHP has its time limit set to 30 seconds. The plugin will attempt to set the time limit to unlimited in order to complete the backup, however if safe mode is enabled this will not be possible. In addition the Apache TimeOut directive has a default of 300 seconds (5 minutes) that cannot be altered without manual intervention. It is not recommended you change this value. However, the plugin has been designed to get around these limitations by using a backup monitor that will detect if the backup has gone away and resume it from where it stopped. In short, this is a feature!