Same problem. From 28 or 29 of July, because I have no fresh incoming emails after then.
iOS mail app: after long refresh it says “Account Error: iCloud details…” on the bottom, when I click ‘details’ it says “The connection to the server failed”.
iOS settings>mail>accounts>iCloud(which goes to Apple ID” page)>icloud>iCloud mail> after long refreshing it says “AUTH_ERROR_INVALID_USR_TOKEN”
iCloud webpage> mail section> “There was a problem loading this content.”
Also iCloud webpage redirects to “sign in” page when I click the ‘custom domain’ settings.
Korean support team failed and senior also didn’t gave me any solution. It was quite embarrassing that they just surprised and couldn’t provide any potential solution.
Wrote the symptoms here because I’m sick and tired of providing the whole story again.
iOS version doesn’t really matter because the problem isn’t just related to the iPhone but same on macOS and iCloud webpage, but NO, I don’t use beta version anywhere with my devices. It’s v14.6.