Exchange 2010: Send As Permission for group mailbox...

Our helpdesk has a shared mailbox used for users to submit issues.  Up until a week ago, all of the helpdesk techs could send-as the shared mailbox.  Now when they attempt to send as the shared mailbox via Outlook they get the error "You do not have the permission to send the message on behalf of the specified user.".  When I attempt to manage send as permission via EMC (using an account with Domain Admin privileges) I've found that the list of users who can send as is blank.  When I attempt to add send as permissions via EMC I get the error below:

domain\username
Failed

Error:
Active Directory operation failed on DC1.xxxxxx.local. This error is not retriable. Additional information: Access is denied.
Active directory response: 00000005: SecErr: DSID-031521E1, problem 4003 (INSUFF_ACCESS_RIGHTS), data 0


The user has insufficient access rights.
Click here for help... http://technet.microsoft.com/en-US/library/ms.exch.err.default(EXCHG.140).aspx?v=14.3.195.1&t=exchgf1&e=ms.exch.err.Ex6AE46B

Exchange Management Shell command attempted:
Add-ADPermission -Identity 'CN=account,DC=xxxxxx,DC=local' -User 'domain\username' -ExtendedRights 'Send-as'

Elapsed Time: 00:00:00

Anyone have any advice to fix this issue?

Thanks in advance for your help.