My app was working fine untder JPATransactionManager, but when I switched to Bitronix, I got this.

The Bitronix guys are saying this is a problem in Hibernate JTA.

STDOUT [WARN ] [2012.02.20 15:14:06] tm.BitronixTransaction - Synchronization.afterCompletion() call failed for org.hibernate.engine.transaction.synchronization.internal.RegisteredSynchronization@405d65c3
java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:373)
at java.util.LinkedHashMap$KeyIterator.next(LinkedHashMap.java:384)
at org.hibernate.engine.transaction.internal.SynchronizationRegistryImpl.notifySynchronizationsAfterTransactionCompletion(SynchronizationRegistryImpl.java:78)
at org.hibernate.engine.transaction.internal.TransactionCoordinatorImpl.sendAfterTransactionCompletionNotifications(TransactionCoordinatorImpl.java:333)
at org.hibernate.engine.transaction.internal.TransactionCoordinatorImpl.afterTransaction(TransactionCoordinatorImpl.java:145)
at org.hibernate.engine.transaction.synchronization.internal.SynchronizationCallbackCoordinatorImpl.afterCompletion(SynchronizationCallbackCoordinatorImpl.java:126)
at org.hibernate.engine.transaction.synchronization.internal.RegisteredSynchronization.afterCompletion(RegisteredSynchronization.java:61)
at bitronix.tm.BitronixTransaction.fireAfterCompletionEvent(BitronixTransaction.java:497)
at bitronix.tm.BitronixTransaction.commit(BitronixTransaction.java:244)
at bitronix.tm.BitronixTransactionManager.commit(BitronixTransactionManager.java:120)
at org.springframework.transaction.jta.JtaTransactionManager.doCommit(JtaTransactionManager.java:1010)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:754)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:723)
at org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.java:393)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:120)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
at org.springframework.aop.framework.Cglib2AopProxy$DynamicAdvisedInterceptor.intercept(Cglib2AopProxy.java:622)
at com.xxxx.ui.administration.users.SiteUserResourceProvider$$EnhancerByCGLIB$$dc04900d.save(<generated>)
at com.xxxx.ui.viewer.AdaptableForm.save(AdaptableForm.java:407)
at com.xxxx.ui.viewer.AdaptableForm.attemptSave(AdaptableForm.java:274)
at com.xxxx.ui.viewer.AdaptableForm$5.buttonClick(AdaptableForm.java:323)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:512)
at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:164)
at com.vaadin.ui.AbstractComponent.fireEvent(AbstractComponent.java:1219)
at com.vaadin.ui.Button.fireClick(Button.java:550)
at com.vaadin.ui.Button.changeVariables(Button.java:217)
at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.changeVariables(AbstractCommunicationManager.java:1451)
at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.handleVariableBurst(AbstractCommunicationManager.java:1399)
at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.handleVariables(AbstractCommunicationManager.java:1318)
at com.vaadin.terminal.gwt.server.AbstractCommunicationManager.doHandleUidlRequest(AbstractCommunicationManager.java:763)
at com.vaadin.terminal.gwt.server.CommunicationManager.handleUidlRequest(CommunicationManager.java:296)
at com.vaadin.terminal.gwt.server.AbstractApplicationServlet.service(AbstractApplicationServlet.java:501)
at com.xxxxx.servlet.ApplicationServlet.service(ApplicationServlet.java:60)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:550)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1359)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:311)
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:116)
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:83)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at com.xxxxx.security.filter.IpAddressAuditFilter.doFilterInternal(IpAddressAuditFilter.java:54)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:113)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:101)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:113)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:54)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:45)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.authentication.www.BasicAuthenticationFilter.doFilter(BasicAuthenticationFilter.java:150)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:182)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:105)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:87)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.session.ConcurrentSessionFilter.doFilter(ConcurrentSessionFilter.java:125)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at com.xxxxxx.security.ConcurrentSessionFilter.doFilter(ConcurrentSessionFilter.java:92)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:323)
at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:173)
at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:237)
at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:167)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1330)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:484)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:119)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:517)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:229)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:970)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:414)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:187)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:904)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:149)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:110)
at org.eclipse.jetty.server.Server.handle(Server.java:347)
at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.java:590)
at org.eclipse.jetty.server.HttpConnection$RequestHandler.content(HttpConnection.java:1071)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:823)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:220)
at org.eclipse.jetty.server.HttpConnection.handle(HttpConnection.java:411)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:535)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:40)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:529)
at java.lang.Thread.run(Thread.java:680)

It could maybe be a Hiberante problem, but fact is:
while your thread who, raised the reported exception, was iterating the map, another thread was either:

-registering for synchronization
or
-calling clearSynchronizations

It seems therefore that in you application different threads are using the same Transaction concurrently which is not allowed.
Can you check that?