The HTTP 500 Internal Server Error is both rare and often difficult to reproduce. It generally indicates a problem with a server somewhere. It may be Elasticsearch, but it could also be a node in the load balancer or proxy. A process restarting is typically the root cause, which means it will often resolve itself within a few seconds.
The easiest solution is to simply catch and retry HTTP 500's. If you've seen this several times in a short period of time, please send us an email and we will investigate.