Windows server 2003 - geplante tasks error 0x80070005 permission denied

if i wanted to add a scheduled taks in windows server 2003 i got an error saying "0x80070005"

permission denied

with the windows explorer it is not possible to change the acl for c:/windows/tasks.

so i had to run cmd.exe an enter

cacls c:\windows\tasks /T /G Administratoren:F

in my group administratoren there are domain-admins included