Can I turn on debug logging of some kind for the repository-s3 plugin?
In 5.3, packages are:
- org.elasticsearch.cloud.aws
 - org.elasticsearch.plugin.repository.s3
 - org.elasticsearch.repositories.s3
 
In 5.6:
- org.elasticsearch.repositories.s3
 
You can change it with:
curl -XPUT 'localhost:9200/_cluster/settings?pretty' -H 'Content-Type: application/json' -d'
{
  "transient": {
    "logger.org.elasticsearch.repositories.s3": "trace"
  }
}
'
Wondering if you could try to upgrade first to latest 5.6?
@Igor_Motov Any other idea?