I think you are running exchange 2007. Exchange 2007 backups are by default snapshot based backup, so even if you do not select AOFO option, Backup Exec will always take a snapshot backup this is by design of Exchange 2007.
Make sure the VSS (volume shadow copy service) on your exchange server is up and running. Also check in the event viewer of the exchange server if there are any events regarding the Volume shadow copy service.
Hope this helps,
Regards,
Tech29