Trying to connect via Safari (Version 14.1.1 (16611.2.5)) to appstoreconnect.apple.com/login, it sits there with a spinner. I can see in the developer console:
[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (app.css, line 0)
[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (common-header.js, line 0)
[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (app.js, line 0)
and those 404'd files:
https://appleid.cdn-apple.com/appleauth/static/cssj/1838721336/widget/auth/app.css
https://appleid.cdn-apple.com/appleauth/static/jsj/1725515964/common-header.js
https://appleid.cdn-apple.com/appleauth/static/jsj/623521622/widget/auth/app.js
I thought it was pulling bad files out of a cache, but wget says (for example):
wget "https://appleid.cdn-apple.com/appleauth/static/jsj/623521622/widget/auth/app.js"
--2021-07-15 09:53:08-- https://appleid.cdn-apple.com/appleauth/static/jsj/623521622/widget/auth/app.js
Resolving appleid.cdn-apple.com (appleid.cdn-apple.com)... 23.3.116.21
Connecting to appleid.cdn-apple.com (appleid.cdn-apple.com)|23.3.116.21|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2021-07-15 09:53:08 ERROR 404: Not Found.
So it looks like it's trying to pull in versions that don't exist.
I've cleared the cache, rebooted, etc. etc. The same happens accessing developer.apple.com It also happens in Safari Technology Preview. It also happens in Opera and Firefox.
It works in Chrome though. I'm using Chrome now to type this in. It actually might be in my version of Chrome's cache ( which I'm not going to clear).
Suffice it to say, this is mysterious and counterproductive.
I am also experiencing the same issue. I was advised by an Apple support specialist to navigate to appleid.apple.com, login there (which is working for me), then while staying logged in, open a different tab and go to appstoreconnect.apple.com. This worked for me, and I am using this workaround for now.