Zookeeper NIOServerCnxnFactory NPE

Trying to figure out what this exception is from zookeeper log.

Background - I am unable to start one of the co-coordinator which has zookeper installed on it. I have another co-coordinator which is starting fine.

This is the warning I am getting from the zookeper log

2017-09-28 16:36:20,125 WARN [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181] org.apache.zookeeper.server.NIOServerCnxnFactory - Ignoring unexpected runtime exception

java.lang.NullPointerException

at org.apache.zookeeper.server.ZooKeeperServer.revalidateSession(ZooKeeperServer.java:591)

at org.apache.zookeeper.server.ZooKeeperServer.reopenSession(ZooKeeperServer.java:605)

at org.apache.zookeeper.server.ZooKeeperServer.processConnectRequest(ZooKeeperServer.java:898)

at org.apache.zookeeper.server.NIOServerCnxn.readConnectRequest(NIOServerCnxn.java:418)

at org.apache.zookeeper.server.NIOServerCnxn.readPayload(NIOServerCnxn.java:198)

at org.apache.zookeeper.server.NIOServerCnxn.doIO(NIOServerCnxn.java:244)

at org.apache.zookeeper.server.NIOServerCnxnFactory.run(NIOServerCnxnFactory.java:203)

at java.lang.Thread.run(Thread.java:745)

Error -

2017-09-28 16:36:21,633 INFO [ProcessThread(sid:0 cport:2181):] org.apache.zookeeper.server.PrepRequestProcessor - Got user-level KeeperException when processing sessionid:0x15ec0a878a50015 type:create cxid:0xa481 zxid:0x1a3874ba txntype:-1 reqpath:n/a Error Path:/druid/discovery/druid:broker/9cf1a8bb-ec77-414e-ae3a-b5ba2c5febdd Error:KeeperErrorCode = NodeExists for /druid/discovery/druid:broker/9cf1a8bb-ec77-414e-ae3a-b5ba2c5febdd

I see that this error had been in the past and there have no issues earlier.