RE: [MULE 2] Work caused exception on 'workCompleted'

In the log, there is only entries like:

<log4j:event logger="org.mule.transport.vm.VMConnector"
                  timestamp="1227190581807"
                  level="ERROR"
                  thread="vmQueue.dispatcher.4">
<log4j:message>
    <![CDATA[Work caused exception on 'workCompleted'. Work being executed was: org.mule.transport.AbstractMessageDispatcher$Worker@1bf836]]>
    </log4j:message>
</log4j:event>

There is not stacktrace dump nor anything else. Just a lot of entries like this one.