There was a problem a couple of weeks ago with the certificate authority that provides the certificate for this site. The issue was resolved overnight though so there shouldn't be any issue now, unless your browser has somehow cached the expired certificate? Try clearing your cache.
Still strange that you are the only one reporting this. I have swapped the site's SSL certificate to a different one anyway, just to be on the safe side, so let me know if it has made a difference (you may need to restart your browser if you get an error just now)
Before you posted the above i was going to post that i was still getting the error, but i couldnt post anything. I have since cleared the cache again and re-booted so here i am. Will it last?
I have six tabs open currently, only this one is throwing up this error