WARN Session 0x0 for server null, unexpected error, closing socket connection and attempting reconnect
I just restarted "zookeeper" and everything went fine.
./ZOOKEEPER_HOME/bin/zkServer.sh restart
"server null" indicates that no value was received for the server parameters while running your ZK CLI.
See if you can override your zookeeper server and port from what's specified in the config file by passing it through the command line.
./zkCli.sh -server hostname:port