Task Scheduler - task does not run

I've created a task to carry out a NAV2007 scan on a daily basis. It runs on demand (i.e. when activated from Task Scheduler) but will not run on schedule.

 

Details are as follows:

 

Task: "C:\Program Files\Norton AntiVirus\Navw32.exe"

Arguments: /TASK:"C:\ProgramData\Symantec\Norton AntiVirus\Tasks\mycomp.sca"

 

Schedule: Daily at 04.00

 

Security Options: Run whether user is logged on or not

 

Conditions: Wake the computer to run this task

 

Settings: Allow task to be run on demand; Run task as soon as possible after a scheduled task has been missed; Stop the task if it runs longer than 1 hour; If the running task does not end when requested, force it to stop

 

The related error messages are as follows:

 

Error 1: Task Scheduler failed to log on "TaskScheduler:Task:{C401E490-64EC-45AA-AF8D-F43E3F873B62}" . Failure occurred in "LogonUser" . User Action: Ensure the credentials for the task are correctly specified. Additional Data: Error Value: 1326.

 

Error 2: Task Scheduler failed to start Task Engine ""  process due to an error occurring in "LUAIsElevatedToken" . Command="taskeng.exe" . Additional Data: Error Value: 2147943726.

 

Error 3: Task Scheduler failed to start "\Norton AntiVirus - Run Full System Scan - Karl (Admin)" task for user "Karl-PC\Karl (Admin)". Additional Data: Error Value: 2147943726.

 

Any ideas?