Delete_by_query conflicts proceed doesn't work in ElasticSearch 5.1.1 with netty4
Side note: This is not the correct way to delete all index data, but I guess you are aware of that.
Can you try to supply the parameter in the restclient as a parameter and not as part of the URL, jsut to make sure that this is not the issue.
I think the main issue here is, that you said you are running your own embedded version of elasticsearch (is this what you mean with in memory). Have you checked that the reindex plugin is installed with that version?