The following is an error message I got when using your plugin with BackupBuddy. It would appear to be "fixable". I really liked the plugin but having reliable backups with a plugin that is a must have takes priority . Please let me know when update becomes available. I would like to reinstall and use.
Error scheduling event with WordPress. Your schedule may not work properly. Please try again. Error #3488439.
This means the built-in WordPress function wp_schedule_event() is returning false. The only thing that can cause that is another plugin or something filtering/blocking scheduling of events. So can try deactivating/disabling all other plugins and then see if it clears it up. If it does then one of them was conflicting and can reactivate the plugins one at a time to find which one causes it again.
Letting the conflicting plugin's author know about the conflict is good to do as well so they can update their plugin and fix it