users failing to initialize means the besadmin account can't access their mailbox, therefore it cannot start the users. if you are certain it's not a perms issue with the info store and/or mailbox then is it possible the user's mailboxes were moved between storage groups with the same mail server? the bes doesn't have the ability to detect those and as a result will faill to start the users.
just out of curiosity have u tried rebooting the bes server?
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