Depending on the configuration, anti-virus applications can be quite debilitating to system and backup performance while Unitrends is performing a backup. In certain cases, backup operations may timeout causing various errors reported by the client and the appliance if on-access/real-time/network scanning is too disruptive to the backup process.
Implement the following exclusions into your A/V policies to prevent false positives and overhead caused by persistent A/V scanning during your backup window.
In addition Microsoft has provided instructions for recommended Antivirus Exclusions on Hyper-V Hosts per MS KB:
Because some antivirus software can quarantine the Unitrends agent when it is listed as a threat, setting the exclusions may sometimes not result in success. In these cases a reinstall of the Unitrends agent following KB 4110 should result in successful backups.