thanks for Mike Smotritsky for reporting this bug!
when only secureClientPort is defined in the config and there is no regular clientPort, then both the stat and the conf commands result in 500 Server Error caused by NullPointerExceptions. The problem is that no serverCnxFactory is defined in the ZooKeeperServer in this case, we have only secureServerCnxnFactory.
see the attached stacktraces about the exceptions (reproduced on the current master branch)
The stat and conf admin commands should actually provide info about both secure and unsecure connections, and should handle the case when any of these are missing.