Backend Error 
This error can occur when migrating to or from G Suite and the API requests being made by CloudMigrator are exceeding Google's rate limits. CloudMigrator 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.