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Solution that worked for me was:
refreshed Plesk components via
Plesk > Server > Components Management > Refresh

Thanks to RackerHacker

DBname normally : psa

I think the error occurs because the run as information is missing and the tasks is corrupted. I was not able to find anything on the forums at Plesk for this error. My only fixed was to delete and recreate the tasks. Oh yeah there is one more problem , i was not able to view the properties for the tasks so i could not see the commands. I add to pull the commands from another working system. i have posted the commands below aliong with the task name. This worked on Plesk version 8.3
All the start in folders should be “C:\Program Files\SWsoft\Plesk\admin” and write down your current schedule for the task and reenter it. The task can run as the system user or the admin .
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Task :
Plesk Scheduler Task #{0C235029-C1F5-4916-AB08-1C6FEA9CE9EA}
Command to execute

C:\WINDOWS\system32\cmd.exe /c “”C:\Program Files\SWsoft\Plesk\admin\bin\runtask.exe” “–application=C:\Program Files\SWsoft\Plesk\admin\bin\php.exe” “–parameters=-q -dauto_prepend_file=\”\” -c php.ini \”C:\Program Files\SWsoft\Plesk\admin\plib\report\autoreport.php\” –auto daily”"

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Task :
Plesk Scheduler Task #{7F9CD2FC-8C81-4f3c-AE0B-BB8C9BA560A7}

Command

C:\WINDOWS\system32\cmd.exe /c “”C:\Program Files\SWsoft\Plesk\admin\bin\runtask.exe” “–application=C:\Program Files\SWsoft\Plesk\admin\bin\php.exe” “–parameters=-q -dauto_prepend_file=\”\” -c php.ini \”C:\Program Files\SWsoft\Plesk\admin\plib\report\autoreport.php\” –auto monthly”"

==================
Task

Plesk Scheduler Task #{99254CDC-8EA7-49ee-8A49-FC2A169843B7}

Command
C:\WINDOWS\system32\cmd.exe /c “”C:\Program Files\SWsoft\Plesk\admin\bin\runtask.exe” “–application=C:\Program Files\SWsoft\Plesk\admin\bin\php.exe” “–parameters=-q -dauto_prepend_file=\”\” -c php.ini \”C:\Program Files\SWsoft\Plesk\admin\plib\report\autoreport.php\” –auto weekly”"
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Task name – stats command
Plesk Scheduler Task #ad08f1e5-f9e2-4be7-b4f7-fc8248acf5e8

command
C:\WINDOWS\system32\cmd.exe /c “”C:\Program Files\SWsoft\Plesk\admin\bin\runtask.exe” –defaultmail “–application=C:\Program Files\SWsoft\Plesk\admin\bin\statistics.exe” “–parameters=”"
==============
Task
Plesk Scheduler Task #C6586631-C086-43FE-9B96-BA28E52FDCD6
Command
C:\WINDOWS\system32\cmd.exe /c “”C:\Program Files\SWsoft\Plesk\admin\bin\runtask.exe” “–application=\”C:\Program Files\SWsoft\Plesk\DrWeb\drwebupw.exe\”" “–parameters= /GO /ST \”/RPC:\Program Files\SWsoft\Plesk\DrWeb\DrWebUpW.log\” \”/DIR:C:\Program Files\SWsoft\Plesk\DrWeb\\\”"”