Using Classic Outlook on Windows for personal email, calendar, and contact management
Have you run into this known bug?
Users get errors 0x800CCC0E & 0x800CCC0F synchronizing Gmail and Yahoo accounts in classic Outlook
Unable to add a Gmail account to classic Outlook
The article title is "Unable to add a Gmail account ..." but I have seen reports that this bug can also cause working GMail accounts to fail. The fix is the same - Clear the Windows Credentials.