Aartjan,
I've seen this board operating and they do tend to get hot, sometimes even to hot to be touched with the finger, but that does not affects normal operation. Also the clock on that board runs quite fast for the time it was designed, so the heat may be just normal. If this is a problem with the board, then probably the best idea would be to let it run until it fails so that you can send it back to NI for repair.
I hope this helps!
Gazoo!
