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. This is the most likely scenario for a Backend Error , but there may be other causes. CloudM Migrate uses exponential backoff to minimise these errors, but on the rare occasion they do cause a failure in migration.
If the issue is caused by rate limiting, 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.