Error 503 while trying to create a S3 snapshot repo

[dr01]

I get a HTTP 503 error whenever I try to create a snapshot repository on an Amazon S3 bucket:

[root@myserver ~]# curl -XPUT 'https://myes.example.org:9200/_snapshot/s3_repository' -H 'Content-Type: application/json' -d '
{
  "type": "s3",
  "settings": { "bucket": "mybucket/elk" }
}'

{"error":{"root_cause":[{"type":"process_cluster_event_timeout_exception","reason":"failed to process cluster event (put_repository [s3_repository]) within 30s"}],"type":"process_cluster_event_timeout_exception","reason":"failed to process cluster event (put_repository [s3_repository]) within 30s"},"status":503}

However, the S3 bucket is reachable from my server:

[root@myserver ~]# aws s3 cp testfile s3://mybucket/elk/ --sse AES256
upload: ./testfile to s3://mybucket/elk/testfile 

Is there some error in the curl command, perhaps?