I've gotten security certificate notifications a number of times, too. I think it started yesterday? I've only accessed DCUM from my Iphone the past few days, so that's been my only experience.
I think the notification says something like, "the security certificate for this website cannot be verified" and then I have the option to click "accept" or "cancel" as to whether I want to proceed to the site or not.
I proceed, of course!