Disabling the antivirus temporarily can help isolate whether the issue is related to interference from the antivirus. Disable\uninstall the antivirus fully and run the Veeam job. If the job works, reenable\reinstall the antivirus, and if the job begins to fail again, the antivirus needs to be investigated.
Many antivirus solutions, such as SentinelOne, have developed modules that monitor and prevent access to the boot configuration data (BCD). These "boot protection modules" have been known 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. If your antivirus solution allows, consider placing the Domain Controllers in a policy or grouping with said boot protection features disabled to prevent them from interfering with backup operations.
For more information about anti-virus exclusions recommended for Veeam Backup & Replication review: https://www.veeam.com/kb1999