Could you please download this app:http://www.sqlbackuprestore.com/downloa ... Reader.zip
Run it on the server where SQL Server is running, and check if the ACEs' for the SQL Server service are adequate to allow the SQL Backup Agent service startup account read access.
Your configuration looks fine. The only permission that the SQL Backup Agent service startup account needs to have is 'Query configuration' (CC), on the SQL Server service.
I would suggest turning on auditing for the 'Object Access' category (e.g. using secpol.msc), run a backup, and check using the Event Viewer which object accesses are being denied.
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