If the file system of a SQL server is backed up but the Backup Exec SQL agent is not licensed/used and the account used for the file backup has no access to SQL, events 18456 may be present from SQL server.
Log Name: Application
When the file backup of a SQL server is initiated, the SQL agent, even if not licensed or used, will query SQL server to determine what databases are active so that their database files can be excluded from the file backup. If the account used for backup has no access to SQL server, this file exclusion information cannot be obtained and SQL server may create events when the access attempt is made.
The account used by Backup Exec for SQL server file backups should have access to all SQL instances present so that the automatic exclusions can be created.