We struggled with the same. The confusing thing is, that creating and selecting files for backup works, whereas the backup itself fails. My findings:
The system logon account is the account, the BE services use. Additionally there are separate logons possible for each ressource, you want to backup. And you can set one account to be used as "default" for all ressources.
BE does not try all logons (see Network/Accounts) if any succeeds for a logon to a ressource. It uses the defined account, the default account, the system logon account, what ever is definded. If onme fails, the backup fails.
Be sure, that the system logon account exists and works. Be sure, that if a default logon is set, this works.
Verify, that the logon accounts for each ressource are correct.
hth Bernd
