Why YSK: after yesterday’s security incident many instances have reset the session tokens which your browser remembers. This is causing issues.
You need to clear cache / cookies / app storage, etc… to eliminate any login problems or problems staying logged in.
I’ve seen many posts mentioning this already.
You’ll probably need to log out and back in from your third-party app. For the browser, https://support.apple.com/en-ca/HT201265.