Strange
Assuming that the drives are compatible, you should be able to insert an old tape, inventory, catalog, restore
Just for grins, try shutting down the BackuExec services, rename the \Catalogs directory (to \Catalogs.new for example) and restart BackupExec
A new, emply \Catalogs directory should be created.
Try the insert, inventory, catalog, select files for restore again
(Remember to swap the production catalog back into place before the next production run)