Same problem happened in July. The quick fix until the Airmail team fixes it, is to put your clock back to Saturday 2nd September, launch Airmail 447 beta, then return your clock back to normal. Just don't quit Airmail or you will have to do all this again. I would say they will have a fix for this in the next few hours or so.
The other good practice missing here is that an update should never be on the day after the expiry date. There should always be 2-3 days overlap even if it means sending out a further copy of the previous one 2-3 days before the expiry with a different date when things go wrong.