Weird recovery situation - not able to open RSG mailboxes using Exmerge

This is a weird situation, so let me explain first:  The system is Exchange 2003 on Windows Server 2003 (both Standard edition). I was requested by a client to recover a mailbox from a backup tape that is 2+ years old. The Exchange server that was backed up onto this tape no longer exists in the organization. The backup was created using Backup Exec 9.1, which the firm is still using.  The software allows you to redirect a RSG restore to a different server, provided that there is a storage group/database on that server with the original name. So, I moved all of the active mailboxes off one of our Exchange servers to the other, deleted the storage group/mailbox store and created a new one named "Mailbox Store (SERVER)", with (SERVER) being the name of the old server. I then created an RSG on this server and was able to redirect the restore from the tape to the RSG.  It appeared to restore successfully, although I had to run a repair on the database after restore.  The database mounts fine, but all of the mailboxes show as "disabled" (i.e., with a red "X").

Now comes the Exmerge part. I'm trying to use Exmerge to export just the single mailbox that I need.  However, Exmerge fails with the message "Error opening message store (MSEMS)" which usually indicates a permissions issue. However, I've double- and triple-checked all of the permissions, and the account I'm using definitely has Full Control permission for both the RSG and the active mailbox database on this server.

One additional quirk of this situation is that the user has an active mailbox currently on a different server than the one I'm using for the recovery.  Is this perhaps causing the issue? I can't create a mailbox for that user on the server that I'm trying to use for the recovery, nor can I move her active mailbox to the recovery server, since one of the requirements of this situation is that the user can't be aware of the fact that this material is being recovered.

Any help/ideas would be greatly appreciated.