This happens because the file in question appears to have no content, which should not happen (meaning, Duplicati should not create such a file).
Does the file exist on S3?
If yes, what size is it? What happens if you remove it from S3 (please keep a copy elsewhere)?
If no, I think the problem is that Duplicati somehow “knows” that the file should be there even though it is not. When it then tries to figure out what contents are in that file, it fails (should be fixed in code then).