Today -- 9/11/15 -- Opera is recognizing the AGO Web site (agohq.org) as protected and the site is displaying properly. I can only assume that this was an anomaly since it was displaying properly up to this week, and now a week later is displaying again as it should. I very much appreciate the comments.

Thanks for your reply. The site is for the American Guild of Organists, so it's not one of those "iffy" sites that are dangerous to access. Opera was displaying the site just fine until last week. Firefox and Chrome both display it. It's a mystery that makes no sense to me. Or to their tech director, whom I've contacted.

Opera's Help site says that an unprotected site has a warning on the right side of the URL space and that by clicking the warning, one can click an "unblock" button. I don't know what to do when that warning does not appear. There must be a way to bypass Opera's refusal to allow a site to display properly, but I don't know what it is. I tried going to the area where you manage security certificates, but I don't really know how to manipulate them.