No mailboxes can be discovered when trying to configure manual scan. Master.mb is empty.
Symantec Mail Security for Microsoft Exchange runs "Get-MailboxDatabase -Status" silently to gather mailboxes details. However, the command needs extra switch "-Server" to run in order to gather mailboxes information properly under Single Node CCR.
This issue has been fixed on version 7.0. Upgrade to version 7.0 or higher to resolve the issue.
Single Node CCR on Microsoft Exchange 2007
Symantec Mail Security for Microsoft Exchange 6.5.x