Safari Won't Open HTTPS Sites

Thanks. Please use the following procedure to determine if the problem is isolated to your Mac, or lies elsewhere.

1. Shut down your Mac.
2. Load macOS Recovery by holding ⌘ and r (two fingers) while you start your Mac with a third finger.
3. At the macOS Utilities screen, release those two fingers.
4. Choose Get Help Online.
5. Safari will launch, but it will lack your bookmarks, favorites, history and other preferences.

Determine if you can open those websites while in that mode.

If the problem does not occur in Recovery mode, but returns after restarting your Mac normally, please read If Safari is slow, stops responding, quits unexpectedly, or has other issues - Apple Support. The applicable section is "Remove Internet plug-ins and other add-ons" and the Support documents that follow "Learn more".

If you remain unable to resolve the problem, please write back.

That site loads just fine on the Mac I'm using, with the same Sierra and Safari versions you posted.

Perform Step 2 in the Discussion link you posted. It seems relevant. So does Step 8. I don't know about the others.

Try running this program in your normal account , then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. On the screen with Options, please open Options and check the bottom 2 boxes before running. Click “Share Report” button in the toolbar, select “Copy Report” and then paste into a reply. This will show what is running on your computer. No personal information is shown.

Etrecheck – System Information

Please read If Safari says it can't establish a secure connection, or the website is using weak encryption - Apple Support.

Needless to say, if you are using any non-Apple "anti-virus" junk, don't.

We have just had this problem too. What cured it was to clear the cache and cookies in Safari and then it worked ok. Hope this helps

Yes this worked for me too.