The "Open File Backup" (VSS) backup job option in NovaBACKUP client, for backup jobs (of any type and selection criteria), will not be able to function on a volume that is larger than 64 TB in size (according to Microsoft directly)

The "Open File Backup" (VSS) backup job option (which is a default enabled backup job option, keep in mind) in NovaBACKUP client, if that option is enabled, to allow for open file backup to function in general, which uses VSS , for backup jobs (of any type and selection criteria), will not be able to function on a volume that is larger than 64 TB in size (according to Microsoft directly). If that is the case that the volume size is larger than 64 TB in size that the file backup job is attempting to backup (using either the legacy or the modern backup engine in the NovaBACKUP client) then it won't be able to take the VSS snapshot with that backup job option enabled, and any open files that are in the backup job selection will have to be skipped and not processed. This will also affect if your file backup job contains any plugins in it, including "System State", and "Hyper-V" and "Microsoft SQL", if for instance the MS SQL database(s) and/or Hyper-V VM(s), besides any files that may be in the backup job selection, are also located on a volume that is 64 TB or larger, for those customers that do not backup purely files in their file backup job in the NovaBACKUP client, but they also backup at least one plugin in the same backup job. To workaround this you could, starting in the 21.2.x client  versions only (as prior client versions did not allow disabling that option, for the  modern backup engine targeted backup jobs only), choose to disable the "Open File Backup" backup job option, by going to the "Backup" tab and then click  on the "Settings" button at the bottom of that dialog, then in the "Backup" tab uncheck the "Open File Backup" backup job option there, and then save that backup job with that change, which will make it so that a VSS snapshot will not be attempted at all, and at the time of the backup job executing if any open files are present they will have to be skipped as files in that backup job run in that case (which may not be reasonable or feasible to have as a disabled backup job option in that case). You could also figure out how to get the files / plugin content on a volume that isn't larger than 64 TB, if that is at all feasible; if the backup job only has files but no plugins in the backup job selection, then the former would be easier to accomplish, as compared to if files + certain plugins, such as "Hyper-V", are in the backup job selection (that in that case you may need to create a separate "Hyper-V" backup job, and a separate other "files" backup job in that case to make that more feasible to do, as to not require to have the files on a volume that is under 64 TB in size, and to have the plugins on a volume that is under 64 TB in size, for a files + plugins backup job, to be able to make all of that work in the end with the "Open File Backup" backup job option enabled, which is a default).

You can read more about that VSS limitation in this Microsoft.com article here (for the line of text in it that states "You run a backup operation on a volume that is larger than 64 TB that has a shadow copy enabled.".