Some users are experiencing an expired certificate error when they attempt to login on the network. This is being caused erroneously by one of our partners (Fastly) and is being actively investigated by them. You can see status updates on their progress here if you're so inclined: https://status.fastly.com/incidents/2tf2hvkj2z50
We apologize for the inconvenience. You may want to try using Firefox in the short term if this is causing you to be unable to participate in comments or write stories.