JMS integration with Nirvana Message Broker

Issue

We have got an installation of a JBoss 5.2 on a RHEL host. We connect to a external JMS queue. The JMS-Queue is situated on a Software AG Nirvana Message Broker. We use the corresponding client libs (ContextFactory: com.pcbsys.nirvana.nSpace.NirvanaContextFactory)

The messages are pure xml and have got a size from about a few kbytes. On JBoss there is a message driven bean that is connected to the queue. What's happening now is as follows
- Messages will be send to the queue
- MDB on JBoss receives them, processes them
- JBoss is commiting the CMT (chnages can be seen in database)
- Anyway after a while (100 messages) the processing stops.
- Shutting down the server is not possible. Thread dump reveals there are still some active threads running nirvana code
- When process is killed, messages are returned to the queue

It seems that there is no ACK send back to nirvana server.

• Is there a known bug in JBoss MDB features that will result in such behaviour?
• Which component is responsible for sending the ACK (JBoss EJB Container, TX-manager or even the jms client framework)?