Hi Tom,
I use Chrome. Which browser are you using? Make sure it is updated. I just had my Chrome browser receive an update the other day. Just a thought. Not sure why a webpage would not display, and others would. But maybe an uninstall and reinstall of your browser would help.
I have not experienced any issues with AVSIM, and can't find the post that Sue quoted. So I can't check on the status of the issue, as I don't see one at all on my system. Weird, huh. Have you tried other browsers, IE or Edge?
Or, ... clear your browser cache, history, and other such traces.