As of 2013 AUG, I have confirmed that Backup Exec 12 REQUIRES that the Backup User account MUST NOT have "Deny Logon Locally" enabled / defined.
Running Backup Exec 12 rev 1364 on a Win2003 R2 server, and having "Deny logon locally" enabled for the Backup User Account, will create the circumstance outlined in KB article Tech129293.
I also confirmed that the backup user account has all of the required Rights assigned, as mentioned in the KB articles:
http://www.symantec.com/business/support/index?page=content&id=TECH129293
http://www.symantec.com/connect/forums/backup-exec-cant-access-local-ressources-list
SYMPTOM: Local and Remote resources are not displayed / inaccessible while editing Backup Selection Lists.
ERROR MESSAGE: Connection with server failed. Hit <F5> to retry.
RESOLUTION: Do not block / deny the right for the backup user account to log on locally.
This is an issue for administrators who wish to improve their system's security posture, because the Backup Operator is able to log into the Domain Controller using their backup user account's credentials. The backup operator is not typically the same person as the systems administrator and, therefore, should not have physical logon access to a DC or any other system, merely to manage backup jobs, selection lists and/or related objects.