I have been using my same gmail account for over a decade now. Always in Mac Mail... never had issues, but recently it just stopped connecting. This icon shows, which you can click, and everything updates. Until the next time I open my MacBook. Connection doctor says no issues with my account (once connected manually). I have removed my account from "Internet Accounts" twice. Neither time has it resolved the issue. I have no idea how to fix and this is very annoying. I suspect it has an old password cached somewhere but both times I added the account back it brought me to gmail (through MacOS) and I added the account, 2FA approved, everything. This account is on my iPhone and works fine.