$ curl -XPUT 10.0.3.160:9200/_snapshot/backup -d '{"type":"s3","settings":{"secret_key":"<super secret key>","access_key":"<access key>","max_restore_bytes_per_sec":"5120mb","bucket":"<my-backup-bucket>","max_snapshot_bytes_per_sec":"5120mb"}}'

{"error":{"root_cause":[{"type":"process_cluster_event_timeout_exception","reason":"failed to process cluster event (put_repository [backup]) within 30s"}],"type":"process_cluster_event_timeout_exception","reason":"failed to process cluster event (put_repository [backup]) within 30s"},"status":503}



from the logs:
[2016-02-11 20:07:15,683][INFO ][rest.suppressed          ] /_snapshot/backup Params: {repository=backup}
ProcessClusterEventTimeoutException[failed to process cluster event (put_repository [backup]) within 30s]
	at org.elasticsearch.cluster.service.InternalClusterService$2$1.run(InternalClusterService.java:343)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)