1st thing to try if you have not already. Delete preferences.
If that does not work then determine if it is a font issue. Disable your font management. Try to open the documents. If they open then you most likely have a corrupt font somewhere. It's also possible that the font you use for the "Normal" style sheet is corrupted. Deleting your prefs should force a default. It's most likely fonts. Usually any OS upgrade ends up reinstalling the base system fonts in the default font folders. This can be a problem if you have fonts in there that you are using already.
If it still does not open it could be an XTension. Pull all of your XTs out of the XTension folder in to a safe place (usually a folder on your desktop). Try to open the documents. If they open then try this. Add back half. Try to open the documents. If they open then you know the problem XT is in the batch you did NOT add back. If it fails to open then the problem XT is in the batch you did add back. Keep experimenting with adding/removing XTs until you find the problem.
If you still can't open these documents without XPress crashing (evem with all XT's removed) then try this. Put the XT's back. Go to the For System folder inside XPress. Double click on the QuarkXPress Components.app and reinstall components. Try and open the documents again.
If none of this works then something something is seriously wrong here. At this point there is not much I can suggest.