UPDATE: the below issue has been resolved.
All. Unfortunately there is an outage at Sendgrid, our email provider, with routing email links. We made a change to our emails to work around the problem for new emails, but the broken links in older ones from the past several hours won't work until they fix things up on their end. Please check your notifications on the web-site for now!
cc/ @ello @wtf @chelsea @markgelband @sean @jayzes @amber @todd @lucian