In my case, this error was reported because Shadow Copy was not enabled on the disks. Because of this BackupExec was unable to connect to the Shadow Copy.
According to Symantec, this can occur if there are one or more servers selected in the backup job which no longer exist on the network. The Job History should be reviewed to determine for which volumes or servers this error was generated. If any of those volumes or servers receiving the error no longer exist, they should be manually removed from the selections of the backup job.