Summarizing my problem again: I'm unable to manually add a JMX Server and got the below stacktrace in the agent side:

2012-03-22 08:32:00,971 INFO  [ResourceDiscoveryComponent.

    invoker.daemon-94] (ems.impl.jmx.connection.DConnection)- Found 154 MBeans - starting load...
    2012-03-22 08:32:00,986 ERROR [WorkerThread#0[10.35.12.74:46690]] (rhq.core.pc.inventory.InventoryManager)- Manual add failed for resource of type [JMX Server] and parent resource id [10001]
    java.lang.reflect.UndeclaredThrowableException
            at $Proxy73.discoverResource(Unknown Source)
            at org.rhq.core.pc.inventory.InventoryManager.discoverResource(InventoryManager.java:357)
            at org.rhq.core.pc.inventory.InventoryManager.manuallyAddResource(InventoryManager.java:687)
            at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
            at java.lang.reflect.Method.invoke(Method.java:597)
            at org.rhq.enterprise.communications.command.impl.remotepojo.server.RemotePojoInvocationCommandService.execute(RemotePojoInvocationCommandService.java:184)
            at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
            at java.lang.reflect.Method.invoke(Method.java:597)
            at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:93)
            at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:27)
            at com.sun.jmx.mbeanserver.MBeanIntrospector.invokeM(MBeanIntrospector.java:208)
            at com.sun.jmx.mbeanserver.PerInterface.invoke(PerInterface.java:120)
            at com.sun.jmx.mbeanserver.MBeanSupport.invoke(MBeanSupport.java:262)
            at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:836)
            at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:761)
            at javax.management.MBeanServerInvocationHandler.invoke(MBeanServerInvocationHandler.java:288)
            at $Proxy0.execute(Unknown Source)
            at org.rhq.enterprise.communications.command.server.CommandProcessor.handleIncomingInvocationRequest(CommandProcessor.java:290)
            at org.rhq.enterprise.communications.command.server.CommandProcessor.invoke(CommandProcessor.java:184)
            at org.jboss.remoting.ServerInvoker.invoke(ServerInvoker.java:809)
            at org.jboss.remoting.transport.socket.ServerThread.processInvocation(ServerThread.java:608)
            at org.jboss.remoting.transport.socket.ServerThread.dorun(ServerThread.java:406)
            at org.jboss.remoting.transport.socket.ServerThread.run(ServerThread.java:173)
    Caused by: java.lang.Exception: Discovery component invocation failed.
            at org.rhq.core.pc.util.DiscoveryComponentProxyFactory$ComponentInvocationThread.call(DiscoveryComponentProxyFactory.java:296)
            at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
            at java.util.concurrent.FutureTask.run(FutureTask.java:138)
            at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
            at java.lang.Thread.run(Thread.java:619)
    Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to [Ljava.lang.Object;
            at javax.management.openmbean.TabularDataSupport.get(TabularDataSupport.java:260)
            at org.rhq.plugins.jmx.JMXDiscoveryComponent.getJavaVersion(JMXDiscoveryComponent.java:267)
            at org.rhq.plugins.jmx.JMXDiscoveryComponent.discoverResource(JMXDiscoveryComponent.java:247)
            at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
            at java.lang.reflect.Method.invoke(Method.java:597)
            at org.rhq.core.pc.util.DiscoveryComponentProxyFactory$ComponentInvocationThread.call(DiscoveryComponentProxyFactory.java:292)
            ... 5 more