Your’re hitting the following error message in your migration logs at the beginning of a migration:
A fatal error occurred in Google Connector for user firstname.lastname@example.org : Precondition failed.: !string.IsNullOrEmpty(authorization.RefreshToken)
This is because the Google account does not exist, is disabled or the user password has expired (in this case Oauth does not work when connecting with a service account).
To solve this problem, change the source password or enable the account.