AlphaSSL intermediate CA certificate revoked
Certificates can be unexpired but still revoked, as shown in your screenshot.

Specifically the issue here seems to be a problem with GlobalSign; a configuration problem caused some browsers to believe that intermediate certificates (such as those issued by AlphaSSL) were revoked when in fact they were not.

As of five days ago, the issue was to work itself out in about four days, as cached responses for intermediate verification expired. Obviously it's not quite there yet. In your case, you can safely check the box to Always trust "AlphaSSL CA - 256SHA - G2", and click Continue.