I tried to do a 400% upscale on a Nikon NEF file. After processing the TIFF file could not be opened by Photoshop or Lightroom. Nor will it open in Windows Photo Viewr (“It looks like we don’t support this format”) I am on a Windows 10 machine with the new 1809 update.
Try and run it again and make sure that the completed image list has the green tick appearing, and don’t use the Clear All button until it has completed. If that is no help please raise a support request at the link above, the completed image list should look like this: