I was ingesting data via Logstash, and I was using the _count API to view how many documents were being ingested. And all of a sudden my Elasticsearch master was unreachable, and it stated that the license cannot be found or not a valid license. I started building this cluster four days ago, so I'm not sure why my license would be an issue. My other data node license seems to be fine.
How many nodes do you have in the cluster? How many nodes are master eligible?
I only have 2 Nodes in total for a PoC. I have 1 master eligible. I got it to eventually work by just restarting. I guess the master node couldn't take all of the request it was being sent.