Problem with validation on an ajax supported h:selectOneMenu

[kairos]

I want to implement a validation on an onchange event of a selectOneMenu. This menu is part of a h:datatable and it must be guaranted that the selected value is not already selected anywhere else in the table. I think this is a situation which might happen quit often. But I am not able to solve it.

This is part of my xhtml:

<h:dataTable id="exTable" var="_exWrapper" value="#{exercises}">
  <h:column>
     <f:facet name="header">#{messages.exName}</f:facet>
     <h:selectOneMenu id="selectExercises" value="#{_exWrapper.exercise}" 
             required="true" converter="#{exerciseConverter}"
          validator="#{myValidator.validate}"
             validatorMessage="#{messages.dublicateExercise}">
         <s:selectItems var="exercise" value="#{exerciseManager.exercises}" 
                label="#{exercise.name}"
                noSelectionLabel="#{messages.pleaseSelect}"/>
             <a4j:support event="onchange" reRender="selectExercise, messagesField" 
                ajaxSingle="true"/>
      </h:selectOneMenu>                          
  </h:column>
....

I tried the validator approach:

@Name("myValidator")
@BypassInterceptors
@org.jboss.seam.annotations.faces.Validator
public class MyGymExerciseValidator implements Validator {

    public void validate(FacesContext arg0, UIComponent arg1, Object arg2)
            throws ValidatorException {
        ExWrapperHome home = (ExWrapperHome) Component.getInstance("exWrapperHome");
        if (!myGymHome.checkExercise((Exercise) arg2)) {
            throw new ValidatorException(new FacesMessage());
        }
    }

}

I created an empty FacesMessage because I didn't succeed in getting access to the resource boundle. Therefore I use the validatorMessage.
Anything works: the validator is called, the exception is thrown but: the problem is that neither message is displayed nor the selectonemenu is rerendered.
But I see the following message in the console:

08:29:26,203 INFO  [lifecycle] WARNING: FacesMessage(s) have been enqueued, but may not have been displayed.
sourceId=myForm:exTable:3:selectExercises[severity=(ERROR 2), summary=(the message), detail=(the message)]

The messages panel is defined in the template.xhtml:

<rich:layout>
    <rich:layoutPanel position="top" width="100%">
     <a4j:outputPanel id="messagesField" ajaxRendered="true">
       <h:messages id="messages" globalOnly="true" styleClass="message"
          errorClass="errormsg" infoClass="infomsg" warnClass="warnmsg"
          rendered="#{showGlobalMessages != 'false'}" />
     </a4j:outputPanel>
     <ui:insert name="body" />
    </rich:layoutPanel>
</rich:layout>

I suppose that after the exception is thrown the reRender of the ajax request does not happen. I tried a lot and searched the www but did not find a hint that helped me.

Any suggestions?

Thanks