Hello,
Check from the tab 'network-logon accounts'. What is the default account specified here? Also check the account used by the Backup exec services. Both these accounts need to be the same. If they aren't, then this kind of error might be produced.
Refer to the following technotes in order to resolve the problem:
Title:How to create a System Logon Account in Backup Exec 9.x and 10.x for Windows Servers
http://support.veritas.com/docs/274012
Title: The error "Your default logon account could not be authenticated" is displayed when attempting to create new jobs or edit existing ones in Backup Exec 9.x and 10.0
http://support.veritas.com/docs/258988
Hope this helps in resolving the problem. If the issue persists, revert with details.
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Regards.