Looks like the BES service account doesn't have the correct permissions to access that user's mailbox.
Is that user a domain admin or a member of any protected AD groups?
A BES user can't be a domain admin.
Before you give me the "well I'm a domain admin and mine works fine" speech... please search the forum. This is one of the most discussed topics for BES Admins.
Remove the user from the domain admins group, and any protected AD groups.
Wait about a half hour, re-apply the Send-As permissions, and you should be fine.
Don't add the user back to the Domain Admins group. It will only fail again eventually.