Google changed their security setup, which does no longer allow you to authenticate against your Google Account password. To fix your issue, you’ll need to go to your account -> security and then to “App-Passwords” (logging in on google). [Link: https://myaccount.google.com/apppasswords?utm_source=google-account ]
As app choose “other” and as name something meaningfull to you, e.g. Zammad", hit generate and copy that pasword into your Zammad password field. Username will remain the same.