WORKAROUND
The following workaround is to be done on the machine running Recovery Manager for Exchange:
1. Create a new Outlook profile for the account with administrative access that can open the Exchange 2003 mailboxes in question
2. Open Outlook with the new profile and if prompted for credentials, enter the credentials and select "remember my credentials"
3. Add new Source Storage in RME
4. Choose Online Exchange 2003 Server
5. When the wizard asks you for credentials type in the same account you added to Windows Credentials Manager.
6. Add the Mailboxes of your choice, and you should be able to expand the mailboxes and restore from them.