Thanks for the additional details. By default the RestHighLevelClient will send type = "MEMORY" and the server will issue a warning. And unfortunately setting type to null will throw an exception.
There are two options:
- use the Java API client for this request. You don't have to migrate your entire application: Java API client and HLRC can smoothly work side by side with the same low level client.
- disable deprecation warnings on the server. But this will hide all deprecation warnings, which may be valuable information.