This seems to be your problem.
Something is preventing your cluster from forming correctly.
The node you are trying to authenticate against cannot access enough master nodes to form a quorum.
The cluster configuration settings were changed in 7.0 to support our new Zen2 discovery & cluster coordination algorithm, and it looks like something has gone wrong with your upgrade to the new settings.
Can you provide your elasticsearch.yml & tell us about how many nodes/what sort of node you have in your cluster?