Ok, digged deeper into the problem and found the cause. As the stacktrace indicates, it's a problem with the MapMessage. Indeed I'm using a MapMessage while pushing it from one queue to another.
But the point is, it seems to be a problem when it's empty:
1) Sending the message via producer to queue A works.
2) Moving the message via MDB on queue A to queue B works.
3) Moving the message via consumer receive (with selector) from B to A again produces the previous mentioned Exception.
Since it was a programming error on my side that the MapMessage wasn't filled, but I think someone might want to take a look at this?