Safari can’t log in to Apple account, icloud.com, Discussions, Apple Store

I cannot log into any Apple services via Safari. Error messages: Connection Error “iCloud encountered an error while trying to connect to the server;” “Safari can’t establish a secure connection to the server idmsa.apple.com.” I can’t log in even in Safe Mode.

I can, however, log in using Firefox, Chrome, and Brave on my Mac, and Safari on iOS 14, with no issues.

I think this happened after I installed Security Update 2020-006 from the App Store.

All systems are up per Apple’s website. No luck after 45 minutes on the phone with an Apple specialist.

Any suggestions appreciated.

High Sierra v. 10.13.6, Safari v. 13.1.2