The class loader used by coherence should also look up in the other plugins.

ClassLoader: org.jivesoftware.openfire.container.PluginClassLoader@199c2e
java.io.IOException: readObject failed: java.lang.ClassNotFoundException: org.jivesoftware.openfire.gateway.registration.cluster.SeniorRegistrationRequest
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:242)
at java.io.ObjectInputStream.resolveClass(ObjectInputStream.java:585)
at com.tangosol.io.ResolvingObjectInputStream.resolveClass(ResolvingObjectInputStream.java:68)
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1544)
at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1466)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1699)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1305)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1908)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1832)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1719)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1305)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
at com.tangosol.util.ExternalizableHelper.readSerializable(ExternalizableHelper.java:2084)
at com.tangosol.util.ExternalizableHelper.readObject(ExternalizableHelper.java:2202)
at com.tangosol.coherence.component.net.Message.readObject(Message.CDB:3)
at com.tangosol.coherence.component.util.daemon.queueProcessor.service.InvocationService$InvocationRequest.read(InvocationService.CDB:8)
at com.tangosol.coherence.component.util.daemon.queueProcessor.Service.onNotify(Service.CDB:110)
at com.tangosol.coherence.component.util.Daemon.run(Daemon.CDB:35)
at java.lang.Thread.run(Thread.java:613)