*UPDATE* - As far as I can tell, this issue is now resolved. Format your USB stick and do the update. If you already updated to the broken version, Garmin Express will notify you that there are no updates available when you insert your newly initialised stick (after putting your car credentials on it again). Ignore this and click the blue 'Map Options' and then click 'Reinstall Map'Please don't perform an update to your Sat Nav system, as described in this thread: viewtopic.php?f=11&t=802 as is will likely render it unusable.
This topic will be updated when Garmin release a confirmed fix.
*UPDATED UPDATE* - IT'S BACK!!!