All my IAPs in the official/non-sandbox version stopped working as well. Also those that do no use an online Apple server check, so I assume the digital receipt already contained those certificates that expire.

It seems to be a broader issue. Since the 1st of October, receipt validation is failing on our iOS app. The path to the receipt is requested by the code but the file isn't there. People have to uninstall and reinstall the app or do an in-app purchase that they have already done so this triggers the 'do you want to get it for free again' requester which is followed by the receipt being reinstated. So far, no one at Apple has listened. Calling 'restore purchases' does not help either.