Have you tried Developer`NotebookConvert[] on it?
https://reference.wolfram.com/language/Developer/ref/NotebookConvert.html
It sounds like you've tried the things suggested at the linked page below, but I include it just in case:
https://mathematica.stackexchange.com/questions/13449/reading-early-version-notebooks
You can open notebooks with a plain-text editor. Notebooks are ASCII files with mark-up that lets them be parsed (by a Front End from an appropriate version of Mathematica). It probably won't help you convert them, but it might help you see where the error message is coming from. You might find other helpful hints, but I doubt it.
