HMaster stuck at "Initialize ServerManager and schedule SCP for crash servers"

Question

Error:

ERROR [RS-EventLoopGroup-1-2] util.NettyFutureUtils (NettyFutureUtils.java:lambda$addListener$0(58)) - Unexpected error caught when processing netty java.lang.IllegalArgumentException: object is not an instance of declaring class..

I am trying to configure "hbase-2.5.6-hadoop3". Hadoop is running fine. Zookeeper is running fine.

HMaster throws only the Netty Error mentioned above. No Error in HRegionServer logs

hbase-site.xml for regionserver is

  <property>
    <name>hbase.cluster.distributed</name>
    <value>true</value>
  </property>
  <property>
    <name>hbase.tmp.dir</name>
    <value>./tmp</value>
  </property>
  <property>
    <name>hbase.unsafe.stream.capability.enforce</name>
    <value>false</value>
  </property>
  <property>
    <name>hbase.master</name>
    <value>lrk1rxxcioaurap02:16000,lrk1rxxcioaurap03:16000</value>
  </property>
  <property>
    <name>hbase.zookeeper.quorum</name>
    <value>lrk1rxxcioaurap02,lrk1rxxcioaurap03</value>
  </property>
  <property>
    <name>hbase.rootdir</name>
    <!-- <value>hdfs://HBaseNamespace/hbase</value> -->
    <value>hdfs://lrk1rxxcioaurap02:8020/hbase</value>
  </property>

  <property>
    <name>hbase.regionserver</name>
    <value>lrk1rxxcioaurap04:16020,lrk1rxxcioaurap05:16020</value>
  </property>
  <property>
    <name>hbase.rpc.timeout</name>
    <value>120000</value> <!-- Set the timeout value in milliseconds -->
  </property>
  <property>
    <name>hbase.master.namespace.init.timeout</name>
    <value>3000000</value>
  </property>
  <property>
    <name>hbase.ssl.allowDefaultHostnameVerifier</name>
    <value>true</value>
  </property>

What could be wrong? Network/ Firewall - Active & Backup Master are interacting & swapping roles when needed. The above error comes on whichever becomes active Master.