This error returns when there is a temporary server failure such as a network connection problem or a server overload.
In general, wait a few seconds and try again.
However, there are two special cases for troubleshooting this error: jobs.get calls and jobs.insert calls.
If you received a 503 error when polling jobs.get, wait a few seconds and poll again.
If the job completes but includes an error object that contains backendError, the job failed. You can safely retry the job without concerns about data consistency.
If you receive this error when making a jobs.insert call, it's unclear if the job succeeded or not. In this situation, you'll need to retry the job.