Yes, this is embarrassing. It happened while I was at work, unable to do anything to fix it and to make things even worse, it happened because I should have added a reminder to my calendar to remind me to renew it before it expired. Sorry about this. Anyways, our new certificate is now live and I have a calendar entry for December 2016 to renew it in good time. Letsencrypt is probably going to be the next thing we use as soon as it becomes mature enough, but for now we'll stick with what we can afford. Once again, my apologies for the confusion and thank you for the emails and for bringing our attention to this.