Does the error page have a code in CAPS_SEPARATED_WITH_UNDERSCORES? There's an article to help decode those, but also, let us know what code you're getting. Note: sometimes you need to click an Advanced button to find the code.

How to troubleshoot security error codes on secure websites

No, it does not. Here is a screenshot of the error message. There is no option to select "Advanced."

Hmm, it's not clear why the connection failed. Might not have anything to do with security settings, certificates, etc.

When this happens, can Firefox connect if you reload the page bypassing the cache? Either:

• Shift+click the Reload button
• Press Command+Shift+R

Do you use a VPN or proxy that routes your connection to the site in a nonstandard way that might look suspicious?

Your suggested commands didn't help as I still get the same error message and it happens regardless of whether or not I'm using VPN. It's become more of a problem, lately.

If you check the Web Console in the tab (Command+Alt+K), does it give any more technical information about the problem?

Is the result any different in a private window (Command+Shift+P)?