I just got a brand new notification from the App Store that this particular update is available, maybe the delivery of the update has been repaired, or other bugs have been worked out of it. Haven't done the update yet yet as I'm mid-project here, but I'll report back once I give it a shot.
***EDIT: Report - it worked this time, laptop was plugged in to power, etc. Not sure if this new release (if it even is a new release of the update) is a cure-all, but it did work for me this time. Here's hoping you get it figured out!