I had the same issue today. All 3 browsers, 2 different OS (El Capitian and Mojave). Tested with help of Apple Support (great people). Went through all of the normal steps to login. Tried each computer/browser w/normal windows, (errors) then tried with "in cognito" or "private windows".
The private windows would load properly with no errors. (making me think it may be some network, cache, issue).
Tried on a different network - normal browser window, was OK. No errors.
To confirm - I quit All browsers, went back to regular network, and tested again, with normal windows, and it all was fine.
Both computers, all Browsers.
Bottom line is: I have no idea why, other than some cache was flushed or something.
--All is fine now--
Any thoughts on what could have been causing it would really be helpful to know.
Thanks. Hope this helps someone else.