If you've recently installed the PAM security update in ubuntu, or have automatic updates enabled, then I recommend reading this: http://www.ubuntu.com/usn/usn-1140-2/ "USN-1140-1 fixed vulnerabilities in PAM. A regression was found that caused cron to stop working with a "Module is unknown" error. As a result, systems configured with automatic updates will not receive updates until cron is restarted, these updates are installed or the system is rebooted. This update fixes the problem." Regards, Graham