access denied when I set "encrypt file to protect offline file"

I have a windows 2003 domain with a server with a shared folder with user
documents.
On one laptop I enable a shared folder to be used off-line. The user has
full control on this folder
To enforce security on laptop I set the flag to encrypt off-line folders,
but when I push "apply" button I receive the message "access denied" ..
Why this happens? How can I use this feature?

Tnak you
Alessandro


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