I was a regular user of the site for years until last year when I suddenly couldn't get access it. I got a similar message to the original poster on this thread no matter which browser I used on both my desktop and laptop. This went on for over 6 months and I gave up. Recently I tried again and managed to access this thread on my Android tablet, I went through all the suggested fixes, and now I can get in occasionally, but it's very hit and miss.
Perhaps this issue only affects a few users, but I wonder how many, like me, gave up on the site thinking it had closed down. Surely if the problem is with the security certificate, the site administrators could solved it