This error occurs when the Veeam Backup Service on the VBR abruptly stops while a backup is running. Most commonly it is because the server is shut down or rebooted. It could also be due to the OS crashing, a power loss event, or the Veeam Backup service crashing. Check the Windows application and system event logs for clues to the source of the issue.
It is always best practice to make sure reboots due to Windows updates or maintenance windows do not interrupt running backups; be sure to schedule updates and maintenance outside of backup windows when possible, and to temporarily disable backup jobs when it is not possible.