File-upload with multiple files fails in Primefaces 5.1

I try to do a file upload in primefaces with PF 5.1 and OmniFaces version 1.7.

My .xhtml code:

<h:form id="fileImportForm" styleClass="prepend-top"
    prependId="false">

    <div style='margin-bottom:10px'>
        <h:outputLabel value="#{msg.file_import_msg}"/>
    </div>
    <p:fileUpload id="fileImport" fileUploadListener="#{fileImport.handleFileUpload}"   
        mode="advanced"  
        update="messages"   
        multiple="true"
        label="#{msg.file_import_choose_label}" 
        uploadLabel="#{msg.file_import_upload_label}" 
        cancelLabel="#{msg.file_import_cancel_label}"       
    />  
    <p:growl id="messages" showDetail="true"/>  

</h:form>

My backing bean:

package test.boundary;

import javax.inject.Named;
import org.primefaces.event.FileUploadEvent;
import com.haslerrail.aura.common.exception.SystemException;

/**
 * BackingBean for <code>fileImport.xhtml</code> page.
 *
 */
@Named
public class FileImport implements java.io.Serializable {

    private static final long serialVersionUID = 1L;

    public void handleFileUpload(final FileUploadEvent event) throws SystemException, InterruptedException {

        if (event == null || event.getFile() == null || event.getFile().getSize() == 0) {
            System.out.println("No file");

        }
        System.out.println(event.getFile().getFileName());
        Thread.sleep(500); // wait a little to force the exception!
    }
}

The problem is, when my handleFileUpload function takes a little bit longer to parse the uploaded file (Thread.sleep(500), it throws an exception! The exception only occurs when I'm uploading more than one file at the same time.

The exception:

09:47:33,171 WARN  [org.jboss.weld.Conversation] WELD-000315 Failed to acquire conversation lock in 1,000 for Conversation with id: 1
09:47:33,180 SEVERE [org.omnifaces.exceptionhandler.FullAjaxExceptionHandler] FullAjaxExceptionHandler: An exception occurred during processing JSF ajax request. Error page '/pages/error/error.xhtml' will be shown.: org.jboss.weld.context.BusyConversationException: WELD-000322 Conversation lock timed out: 1
    at org.jboss.weld.context.AbstractConversationContext.activate(AbstractConversationContext.java:215) [weld-core-1.1.5.AS71.Final.jar:2012-02-10 15:31]
    at org.jboss.weld.jsf.WeldPhaseListener.activateConversations(WeldPhaseListener.java:108) [weld-core-1.1.5.AS71.Final.jar:2012-02-10 15:31]
    at org.jboss.weld.jsf.WeldPhaseListener.beforePhase(WeldPhaseListener.java:85) [weld-core-1.1.5.AS71.Final.jar:2012-02-10 15:31]
    at com.sun.faces.lifecycle.Phase.handleBeforePhase(Phase.java:228) [jsf-impl-2.1.7-jbossorg-2.jar:]
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:99) [jsf-impl-2.1.7-jbossorg-2.jar:]
    at com.sun.faces.lifecycle.RestoreViewPhase.doPhase(RestoreViewPhase.java:116) [jsf-impl-2.1.7-jbossorg-2.jar:]
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118) [jsf-impl-2.1.7-jbossorg-2.jar:]
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.primefaces.webapp.filter.FileUploadFilter.doFilter(FileUploadFilter.java:102) [primefaces-5.1.jar:5.1]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.jboss.weld.servlet.ConversationPropagationFilter.doFilter(ConversationPropagationFilter.java:62) [weld-core-1.1.5.AS71.Final.jar:2012-02-10 15:31]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.omnifaces.filter.CacheControlFilter.doFilter(CacheControlFilter.java:226) [omnifaces-1.7.jar:1.7]
    at org.omnifaces.filter.HttpFilter.doFilter(HttpFilter.java:77) [omnifaces-1.7.jar:1.7]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:489) [jbossweb-7.0.13.Final.jar:]
    at org.jboss.as.jpa.interceptor.WebNonTxEmCloserValve.invoke(WebNonTxEmCloserValve.java:50) [jboss-as-jpa-7.1.1.Final.jar:7.1.1.Final]
    at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153) [jboss-as-web-7.1.1.Final.jar:7.1.1.Final]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368) [jbossweb-7.0.13.Final.jar:]
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [jbossweb-7.0.13.Final.jar:]
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:671) [jbossweb-7.0.13.Final.jar:]
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930) [jbossweb-7.0.13.Final.jar:]
    at java.lang.Thread.run(Thread.java:745) [rt.jar:1.7.0_75]  

For me it looks like an error in PF 5.1! Any other ideas?