Nas 1200S sometimes defrag failed

[Gasulla]

Hello,
Sometimes the automatic defrag on the disk F: was failed . The disk F is in raid 5.
In teh event viewer I had:
Disk defragmentation of Volume F: could not completed because the account used to run the defragmentation is not an administrator, there is a problem loading the resource DLL, or a Defragmentation engine could not be accessed. Check for proper user permissions and run Sfc.exe to validate system files (Exit code 6).
A hard disk was have some problems?

Thanks for your response