Same kind of problem, using JBoss 4.0.5.GA. Started when I had a new application scoped SFSB in Seam. Very long and in a few minutes recurring stack trace begins with 00:04:03,187 ERROR [SimpleStatefulCache] problem passivation thread | javax.ejb.EJBException: Could not passivate; failed to save state | at org.jboss.ejb3.cache.simple.StatefulSessionFilePersistenceManager.passivateSession(StatefulSessionFilePersistenceManager.java:363) | at org.jboss.ejb3.cache.simple.SimpleStatefulCache.passivate(SimpleStatefulCache.java:196) | at org.jboss.ejb3.cache.simple.SimpleStatefulCache$SessionTimeoutTask.run(SimpleStatefulCache.java:129) | Caused by: java.io.IOException | at org.jboss.serial.persister.RegularObjectPersister.writeSlotWithMethod(RegularObjectPersister.java:129) | at org.jboss.serial.persister.RegularObjectPersister.defaultWrite(RegularObjectPersister.java:86) | at org.jboss.serial.persister.RegularObjectPersister.writeData(RegularObjectPersister.java:62) | at org.jboss.serial.objectmetamodel.ObjectDescriptorFactory.describeObject(ObjectDescriptorFactory.java:276) | at org.jboss.serial.objectmetamodel.DataContainer$DataContainerDirectOutput.writeObject(DataContainer.java:206) and ends with Caused by: java.lang.NoClassDefFoundError: org/jaxen/VariableContext | at java.lang.Class.getDeclaredMethods0(Native Method) | at java.lang.Class.privateGetDeclaredMethods(Class.java:2395) | at java.lang.Class.getDeclaredMethod(Class.java:1907) | at org.jboss.serial.classmetamodel.ClassMetaData.lookupMethodOnHierarchy(ClassMetaData.java:102) | at org.jboss.serial.classmetamodel.ClassMetaData.lookupInternalMethods(ClassMetaData.java:432) | at org.jboss.serial.classmetamodel.ClassMetaData.<init>(ClassMetaData.java:122) | at org.jboss.serial.classmetamodel.ClassMetamodelFactory.getClassMetaData(ClassMetamodelFactory.java:350) | at org.jboss.serial.objectmetamodel.ObjectDescriptorFactory.describeObject(ObjectDescriptorFactory.java:168) | at org.jboss.serial.objectmetamodel.DataContainer$DataContainerDirectOutput.writeObject(DataContainer.java:206) | at org.jboss.serial.persister.ArrayPersister.saveObjectArray(ArrayPersister.java:110) | at org.jboss.serial.persister.ArrayPersister.writeData(ArrayPersister.java:101) | at org.jboss.serial.objectmetamodel.ObjectDescriptorFactory.describeObject(ObjectDescriptorFactory.java:276) | at org.jboss.serial.objectmetamodel.DataContainer$DataContainerDirectOutput.writeObject(DataContainer.java:206) | at org.jboss.serial.persister.RegularObjectPersister.writeSlotWithFields(RegularObjectPersister.java:182) | at org.jboss.serial.persister.ObjectOutputStreamProxy.writeFields(ObjectOutputStreamProxy.java:79) | at org.jboss.serial.persister.ObjectOutputStreamProxy.defaultWriteObject(ObjectOutputStreamProxy.java:68) | at java.util.Vector.writeObject(Vector.java:1018) | ... 132 more Seems like it cannot passivate. Jaxen is present in JBoss. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4022907#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...