I'm not sure we can really provide any help, since you're using the Zookeeper plugin. Basically, the error says the node couldn't find a new master node to talk to. Master detection and discovery are normally controlled via the internal Zen service, but the Zookeeper plugin replaces it with Zookeeper.
Replacing something as large/complicated as discovery in Elasticsearch with an external service is bound to be difficult and/or buggy. It may have been a transient network blip, or it may have been some bug in the unofficial plugin. It's hard to say since we don't maintain it.
I'd say open a ticket with the maintainers of the plugin, or revert back to the regular Zen discovery service.