Update: This issue has been fixed.
I updated my iMac to macOS 10.4.4 yesterday. This morning, when I went into my home office to work, I found that I needed to log into my Google account (I have my personal domain hosted on a Gsuite account) in order to get email.
I clicked Open Safari, and Safari opened a private browsing window with a login page for Google. I entered my email address and password, then a one-time code, and it bounced me back to Mail, but all I saw was this:
Something wasn’t sending the appropriate token back to Mail to allow the login. (I later tried creating an app-specific password for Mail on Google, but that was refused; it looked like the login it was requesting was not specific to Mail, but to the Google account in general.)
This was only happening on my iMac, which I had updated to macOS 10.4.4; not my MacBook Pro which hadn’t been updated, nor my iPhone. I called AppleCare, and they had me create a new user account on the iMac, where the same thing happened, confirming that the problem has something to do with this Mac, rather than my user account.
One thing that surprises me is this authentication procedure via Safari. In the past, I’m sure I entered my Google password in Mail, or in the Internet Accounts pane of System Preferences, and this Apple support document shows that you enter your password in Mail. So I’m not sure if this new process is causing the problem, or, as the Apple Care senior support person said, it has something to do with a certificate.
This is, of course, quite problematic. If you rely on email for your work, and can’t get your email, you are hindered. I can get the email on my other devices, or I can use the Gmail web interface, so I’m not totally locked out, but still; I manage my email in the Mail app, and don’t like using the web.
This sort of issue raises the question of whether it’s really safe to use an email system that does not work with standard username/password authentication. For some reason, the login requires transiting via a web browser, which means there is a weak link that can break.
In any case, I’m throwing this out in case anyone else has this problem. The Apple Care technician said that he was sending this to engineering as a high-priority issue, and doesn’t think that a macOS update would be required, that he thinks it’s most likely a certificate that may simply need to be updated. I’ll post more info when I get it.
Update: There’s an Apple forum thread about this issue, so it’s clearly a problem affecting a lot of people. Some users are reporting that this was a bug in the beta version of macOS 10.4.4, which was duly reported by many testers. If so, Apple really messed up, because so many people use Google for their email.
Some users are able to get their accounts to work by deleting the existing Google account, then re-creating it as an IMAP account, but this means that it isn’t recognized as a full Google account, which is problematic for those who use Google for calendars and other services.