Passivation problem with Bean race session

I use JBoss 6.1 Final and get the following error message after my web application has been working for some time (note that the application is not crashing), followed by a very long stack trace. I notice that this problem only occurs in a session with beans state, in which another session with beans state is entered into it.

16: 10: 59,769 ERROR [org.jboss.ejb3.cache.simple.SimpleStatefulCache.UutSerialNumberServiceBean] problem of passivation problem: javax.ejb.EJBException: failed to passivate; failed to save state

Here is an abusive session with a bean state ...

package com.ray.TEI.ejb;

import java.io.Serializable;

import javax.ejb.Stateful;
import javax.inject.Inject;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceContextType;

import com.ray.TEI.model.TestProcedure;
import com.ray.TEI.model.Uut;
import com.ray.TEI.model.UutSerialNumber;

@Stateful
public class UutSerialNumberServiceBean implements Serializable {
  private static final long serialVersionUID = 1L;
  @PersistenceContext(unitName="jasoni", type=PersistenceContextType.EXTENDED)
  protected EntityManager em;
  @Inject private ExecProcedureServiceBean execProcedureServiceBean;

  public boolean isDuplicateSerialNumber(Uut uut, String serialNumber) {
    return ((Number)em.createNamedQuery("UutSerialNumber.getCountByUutIdAndSerialNumber")
             .setParameter("uut", uut)
             .setParameter("serialNumber", serialNumber)
             .getSingleResult()).intValue() > 0;
  }

  public UutSerialNumber findUutSerialNumberByUutSerialNumberId(Integer uutSerialNumberId) {
    return em.find(UutSerialNumber.class, uutSerialNumberId);
  }

  public UutSerialNumber editSerialNumber(Integer uutSerialNumberId, String serialNumber) {
    UutSerialNumber uutSerialNumber = findUutSerialNumberByUutSerialNumberId(uutSerialNumberId);
    uutSerialNumber.setSerialNumber(serialNumber);
    return uutSerialNumber;
  }

  public UutSerialNumber createSerialNumber(Uut uut, String serialNumber) {
    UutSerialNumber uutSerialNumber = new UutSerialNumber();
    uutSerialNumber.setSerialNumber(serialNumber);
    uutSerialNumber.setUut(uut);
    uut.getSerialNumbers().add(uutSerialNumber);
    em.persist(uutSerialNumber);
    for (TestProcedure testProcedure : uut.getTestProcedures()) {
      execProcedureServiceBean.createExecProcedure(uutSerialNumber, testProcedure);
    }
    return uutSerialNumber;
  }
}

Here is an incremental session with a bean state ...

package com.ray.TEI.ejb;

import java.io.Serializable;

import javax.ejb.Stateful;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceContextType;

import com.ray.TEI.model.ExecProcedure;
import com.ray.TEI.model.TestProcedure;
import com.ray.TEI.model.UutSerialNumber;


@Stateful
public class ExecProcedureServiceBean implements Serializable {
  private static final long serialVersionUID = 1L;
  @PersistenceContext(unitName="jasoni", type=PersistenceContextType.EXTENDED)
  protected EntityManager em;

  public ExecProcedure createExecProcedure(UutSerialNumber uutSerialNumber, TestProcedure testProcedure) {
    ExecProcedure execProcedure = new ExecProcedure();
    execProcedure.setUutSerialNumber(uutSerialNumber);
    execProcedure.setTestProcedure(testProcedure);
    execProcedure.setIterationCount(0);
    em.persist(execProcedure);
    return execProcedure;
  }
}

Does anyone know what's wrong with that?

Thanks Jason