Trying to expand the SQL Server Instance in the backup selection list will fail. Any previously created backups will fail with "Unable to attach to a resource."
"An attempt to connect to SQL Server has failed. The logon account may have invalid credentials, or the logon account may have been applied to the wrong resource."
This can be caused by the Backup Exec account not having the correct credentials in the SQL Instance.
2. Also add the Backup Exec account to local administrator group on SQL server .