Backend Error 
This error can occur when migrating to or from Google Workspace and the API requests being made by CloudM Migrate are exceeding Google's rate limits. CloudM Migrate uses exponential backoff to minimise these errors, but on the rare occasion they do cause a failure in migration, the recommended solution is to simply restart the migration. After a period (up to 24 hours) the Google limits will be reset and the migration will succeed. For further information on API quotas and throttling see here.