Scheduled tasks problems can be tough. Did you look under Control Panel, Scheduled Tasks or under Event Viewer to see the details as to why the task failed?
Be absolutely certain that the username used when the scheduled task runs does not use an blank or empty password.
For NAV tasks, use Control Panel, Scheduled Tasks and remove the entries. Use NAV itself to rebuild the entries. Then edit through the Scheduled Task applet the Log On to provide logon credentials equal to the local Administrator or a member of the Backup group. It is possible that your NAV assignments included mapped drives, and your are failing on a permissions issue due to this. Define a Log On user that has permissions to mapped or shared folders, or exclude those under NAV itself as part of the search. The default permissions under Scheduled Tasks would by your local SYSTEM account, and it has no privileges to access network resources.