Disabling the anti-virus temporarily can help isolate whether the issue is related to interference from the anti-virus. Disable\uninstall the anti-virus fully and run the Veeam job. If the job works, reenable\reinstall the antivirus, and if the job begins to fail again, the anti-virus needs to be investigated.
Many anti-virus solutions have developed modules that monitor and prevent access to the boot configuration data (BCD). These "boot protection modules" have been observed to prevent Veeam's Application-Aware Processing processing from working with Domain Controllers. During the backup job's Application-Aware Processing step, for Domain Controllers only, the BCD is temporarily modified to enable SafeBoot.
For more information about anti-virus exclusions recommended for Veeam Backup & Replication review: https://www.veeam.com/kb1999