Recently I had problems by upgrading Veeam from 9.5U4 to v10. Upgrade process failed after a while. We found out, directory C:\Program Files\Veeam\Backup and Replication\Backup lost its permissions. We had no change any more to enter this directory.
Short answer: Antivirus software causes the problem. After disabling real-time scan, upgrade run without further issues. Antivirus software was configured according Veeam best practices.