The specified network name is no longer available. V-79-57344-34036 - An unknown error has occurred.

This could be because your backup selection list contains something that no longer exists because it has been decommissioned. 12.5 is a bit old but somewhere in the screen where you choose the backup selections should be a details view (or text view) that you can use to check what the exact selection entries are (as the tree view may not show something that is no longer online even though the selection is still in the job)

If you select a higher level directory then whateve additions or deletions beneath that level would automaticallybe taken care of.  For example, if you select C:\ then whatever additions/deletions under C:\ would be taken care of.  If you select C:\a, C:\b and so on, and you delete C:\b.  You would need to manually delete C:\b from your selection.  Likewise if you add C:\b1, you have to manually select this new directory.