Yes I somtimes get that message when a file is made using a previous version of ITensor. We try to keep backwards compatibility of the file formats but currently the only guarantee is that files read using the same version of ITensor, on the same computer, are guaranteed to work correctly. ("Guaranteed" means what we consider a bug or not.)
So is it possible you wrote the file with a previous version of ITensor? If so that could be the culprit.
If not, can you tell me how you created the file? Like did you make it on a different computer possibly?