Safari can't open Apple's own Discussions Sign In Page!

Does this work when booted from Safe Mode? That’ll disable certain sorts of add-ons.

Tried clearing the web site data (cookies) for Apple sites?

Any add-on anti-malware, add-on anti-virus, add-on cleaners, add-on firewalls, add-on VPN clients, or such, installed here?

MikeV2020 wrote:

I also am unable to connect to iCloud.com using the Safari browser - other browsers work fine.

Does this work when booted from Safe Mode? That’ll disable certain sorts of add-ons.

Tried clearing the web site data (cookies) for Apple sites?

Any add-on anti-malware, add-on anti-virus, add-on cleaners, add-on firewalls, add-on VPN clients, or such, installed here?

Download and enable full device access and run EtreCheck, and open a new reply here and then press the button that looks like a printed page to get a text input box big enough to paste the hardware and software configuration report here.

The message says:

Safari can’t establish a secure  connection to the server?

1. Make sure Date and Date is set  to automatically.

   If already checked, uncheck and recheck it.

https://support.apple.com/en-us/HT203413

2 Disable or uninstall anti-virus software if installed on your Mac.

There is now a fix (or workaround) for it found by a very clever person:

https://discussions.apple.com/thread/251211674

It’s not High Sierra generically. macOS 10.13.6 authenticates with ASC just fine on a test system here, FWIW. That High Sierra box did get caught up in the cookie mess a while back, and all web site data was cleared. If it’s not stale cookies or a stale DNS translation, then this could well be an issue with a subset of Apple servers, or a subset of Apple DNS.

If you are running High Sierra as your profile shows, others are having the same problem. See the discussion below.

Cannot sign in to Apple Community using S… - Apple Community

You might check macjack's post in Community Operations.

After everything those affected have tried in the Community Operations thread and myself troubleshooting with the engineers, I think you are correct about Apple servers or Apple DNS. But even if they do fix it, we'll never learn what it was.

There's nothing you can do on your end to troubleshoot this. Except to upgrade if possible. Use another browser like Firefox and let's hope it gets sorted soon.

I also am unable to connect to iCloud.com using the Safari browser - other browsers work fine.