When attempting to perform a Vault Cache / Virtual Vault synchronization, Event ID 6941 is reported in the Enterprise Vault server event log stating that the user does not have the Control Folder permission. A review of permissions within Exchange and the Enterprise Vault Administration Console shows that the user should have full access to the archive, and therefore, should have this permission.
Event ID: 6941
Client request refused due to insufficient privileges, user <userID> attempted to access the Archive Folder <folderID> but does not have permission(s) (Control folder).
A frequent cause of this Warning is a user attempting an operation on a forwarded, moved, or copied shortcut to a Vault for which they do not have the required permission(s).
As items are archived from various folder locations within a user mailbox, these folder locations will remain within the Enterprise Vault databases even if the folders were removed from the mailbox. Subsequently, these 'orphaned' folders within the database may not have proper permissions synchronized from Exchange as these folders and any subsequent child folders no longer exist in the mailbox. The 'CONTROL_FOLDER' permission does not synchronize to the folder in the Enterprise Vault database, causing this error.