ES won't start up at all due to a Java issue

Elastic Stack Elasticsearch
1

java.lang.IllegalArgumentException: Plugin [delete-by-query] is incompatible with Elasticsearch [2.4.0]. Was designed for version [2.3.5]

What do I do? Best to downgrade back to 2.3.5 until 2.4.0 becomes more stable?

2

Upgrade the dele by query plugin. That's all what you need.