Enterprise Vault - Issues Restoring

I have one user who is having problems restoring from a delgate Mailbox he has Full Mailbox Access to.  Whenever he tries to restore an email he gets the error:

'Could not process the selected items - Reason: A Server Error Has Occurred'

Other users with access to this Mailbox are able to restore items without issue.  The affected user can restore items to his own Mailbox without issue.

I have checked all Mailbox permissions, I have added the persmissions specifically on the Vault side and I have a run a Vault Synch on his account but nothing has worked.

Does anyone have any suggestions at all please?