I received the same error starting the minimal confiuguration of JBoss 4.2.0.GA using java 1.6.0 and then trying to use JConsole to access the process via JMX.
12:08:54,515 INFO [Server] Starting JBoss (MX MicroKernel)...
12:08:54,531 INFO [Server] Release ID: JBoss [Trinity] 4.2.0.GA (build: SVNTag=JBoss_4_2_0_GA date=200705111440)
12:08:54,531 INFO [Server] Home Dir: C:\opt\jboss-4.2.0.GA
12:08:54,531 INFO [Server] Home URL: file:/C:/opt/jboss-4.2.0.GA/
12:08:54,531 INFO [Server] Patch URL: null
12:08:54,546 INFO [Server] Server Name: minimal
12:08:54,546 INFO [Server] Server Home Dir: C:\opt\jboss-4.2.0.GA\server\minimal
12:08:54,546 INFO [Server] Server Home URL: file:/C:/opt/jboss-4.2.0.GA/server/minimal/
12:08:54,546 INFO [Server] Server Log Dir: C:\opt\jboss-4.2.0.GA\server\minimal\log
12:08:54,562 INFO [Server] Server Temp Dir: C:\opt\jboss-4.2.0.GA\server\minimal\tmp
12:08:54,562 INFO [Server] Root Deployment Filename: jboss-service.xml
12:08:54,843 INFO [ServerInfo] Java version: 1.6.0_01,Sun Microsystems Inc.
12:08:54,859 INFO [ServerInfo] Java VM: Java HotSpot(TM) Server VM 1.6.0_01-b06,Sun Microsystems Inc.
12:08:54,859 INFO [ServerInfo] OS-System: Windows XP 5.1,x86
12:08:55,109 INFO [Server] Core system initialized
12:08:55,562 INFO [Log4jService$URLWatchTimerTask] Configuring from URL: resource:jboss-log4j.xml
12:08:55,843 INFO [Server] JBoss (MX MicroKernel) [4.2.0.GA (build: SVNTag=JBoss_4_2_0_GA date=200705111440)] Started in 1s:281ms
12:09:06,500 ERROR [STDERR] javax.management.JMRuntimeException: Failed to load MBeanServerBuilder class org.jboss.mx.server.MBeanServerBuilderImpl: java.lang.ClassNotFoundException: org.jboss.mx.server.MBeanS
12:09:06,515 ERROR [STDERR] at javax.management.MBeanServerFactory.checkMBeanServerBuilder(MBeanServerFactory.java:480)
12:09:06,515 ERROR [STDERR] at javax.management.MBeanServerFactory.getNewMBeanServerBuilder(MBeanServerFactory.java:511)
12:09:06,515 ERROR [STDERR] at javax.management.MBeanServerFactory.newMBeanServer(MBeanServerFactory.java:298)
12:09:06,515 ERROR [STDERR] at javax.management.MBeanServerFactory.createMBeanServer(MBeanServerFactory.java:213)
12:09:06,515 ERROR [STDERR] at javax.management.MBeanServerFactory.createMBeanServer(MBeanServerFactory.java:174)
12:09:06,515 ERROR [STDERR] at sun.management.ManagementFactory.createPlatformMBeanServer(ManagementFactory.java:302)
12:09:06,515 ERROR [STDERR] at java.lang.management.ManagementFactory.getPlatformMBeanServer(ManagementFactory.java:504)
12:09:06,515 ERROR [STDERR] at sun.management.jmxremote.ConnectorBootstrap.startLocalConnectorServer(ConnectorBootstrap.java:412)
12:09:06,515 ERROR [STDERR] at sun.management.Agent.startAgent(Agent.java:135)
12:09:06,515 ERROR [STDERR] at sun.management.Agent.agentmain(Agent.java:95)
12:09:06,515 ERROR [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
12:09:06,531 ERROR [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
12:09:06,531 ERROR [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
12:09:06,531 ERROR [STDERR] at java.lang.reflect.Method.invoke(Method.java:597)
12:09:06,531 ERROR [STDERR] at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:325)
12:09:06,531 ERROR [STDERR] at sun.instrument.InstrumentationImpl.loadClassAndCallAgentmain(InstrumentationImpl.java:348)
12:09:06,531 ERROR [STDERR] Caused by: java.lang.ClassNotFoundException: org.jboss.mx.server.MBeanServerBuilderImpl
12:09:06,531 ERROR [STDERR] at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
12:09:06,531 ERROR [STDERR] at java.security.AccessController.doPrivileged(Native Method)
12:09:06,531 ERROR [STDERR] at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
12:09:06,531 ERROR [STDERR] at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
12:09:06,531 ERROR [STDERR] at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
12:09:06,531 ERROR [STDERR] at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
12:09:06,546 ERROR [STDERR] at javax.management.MBeanServerFactory.loadBuilderClass(MBeanServerFactory.java:423)
12:09:06,546 ERROR [STDERR] at javax.management.MBeanServerFactory.checkMBeanServerBuilder(MBeanServerFactory.java:465)
12:09:06,546 ERROR [STDERR] ... 15 more
12:09:06,546 ERROR [STDERR] Agenten orsakade ett undantag : javax.management.JMRuntimeException: Failed to load MBeanServerBuilder class org.jboss.mx.server.MBeanServerBuilderImpl: java.lang.ClassNotFoundExcep
Exception in thread "Attach Listener" 12:09:06,546 ERROR [STDERR] java.lang.reflect.InvocationTargetException
12:09:06,546 ERROR [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
12:09:06,546 ERROR [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
12:09:06,546 ERROR [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
12:09:06,546 ERROR [STDERR] at java.lang.reflect.Method.invoke(Method.java:597)
12:09:06,546 ERROR [STDERR] at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:325)
12:09:06,546 ERROR [STDERR] at sun.instrument.InstrumentationImpl.loadClassAndCallAgentmain(InstrumentationImpl.java:348)
12:09:06,546 ERROR [STDERR] Caused by: java.lang.RuntimeException: javax.management.JMRuntimeException: Failed to load MBeanServerBuilder class org.jboss.mx.server.MBeanServerBuilderImpl: java.lang.ClassNotFou
12:09:06,562 ERROR [STDERR] at sun.management.Agent.error(Agent.java:309)
12:09:06,562 ERROR [STDERR] at sun.management.Agent.startAgent(Agent.java:153)
12:09:06,562 ERROR [STDERR] at sun.management.Agent.agentmain(Agent.java:95)
12:09:06,562 ERROR [STDERR] ... 6 more
12:09:06,562 ERROR [STDERR] Caused by: javax.management.JMRuntimeException: Failed to load MBeanServerBuilder class org.jboss.mx.server.MBeanServerBuilderImpl: java.lang.ClassNotFoundException: org.jboss.mx.se
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.checkMBeanServerBuilder(MBeanServerFactory.java:480)
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.getNewMBeanServerBuilder(MBeanServerFactory.java:511)
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.newMBeanServer(MBeanServerFactory.java:298)
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.createMBeanServer(MBeanServerFactory.java:213)
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.createMBeanServer(MBeanServerFactory.java:174)
12:09:06,562 ERROR [STDERR] at sun.management.ManagementFactory.createPlatformMBeanServer(ManagementFactory.java:302)
12:09:06,562 ERROR [STDERR] at java.lang.management.ManagementFactory.getPlatformMBeanServer(ManagementFactory.java:504)
12:09:06,562 ERROR [STDERR] at sun.management.jmxremote.ConnectorBootstrap.startLocalConnectorServer(ConnectorBootstrap.java:412)
12:09:06,562 ERROR [STDERR] at sun.management.Agent.startAgent(Agent.java:135)
12:09:06,562 ERROR [STDERR] ... 7 more
12:09:06,562 ERROR [STDERR] Caused by: java.lang.ClassNotFoundException: org.jboss.mx.server.MBeanServerBuilderImpl
12:09:06,562 ERROR [STDERR] at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
12:09:06,562 ERROR [STDERR] at java.security.AccessController.doPrivileged(Native Method)
12:09:06,562 ERROR [STDERR] at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
12:09:06,562 ERROR [STDERR] at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
12:09:06,562 ERROR [STDERR] at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
12:09:06,562 ERROR [STDERR] at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.loadBuilderClass(MBeanServerFactory.java:423)
12:09:06,562 ERROR [STDERR] at javax.management.MBeanServerFactory.checkMBeanServerBuilder(MBeanServerFactory.java:465)
12:09:06,562 ERROR [STDERR] ... 15 more
Agent failed to start!
Any hints of what may be wrong