You need to check where NetBackup is installed. PBX is a separate package.
The default install path for NBU is C:\program files\veritas\netbackup. It seems that the Notes client also has a D:\program files folder. Else use the search utility in Windows Explorer.
Please create the bpbkar folder under the netbackup\logs directory. Log file will be created next time the backup runs.
The service logon account seems to be fine.
We will need bpbkar log and Event viewer Application log as well.
The NBU backup process is merely reporting the Windows error. So, the Application log will possibly be the place to find most helpful information.