I open Constraints Editor from the ISE GUI, go to the "Misc" tab, and select the IOB Register Control button. I add a latch from the "Available Registers" window to the "Registers for IOB Packing" window, and press either "Apply" or "OK". At this point, the Constraints Editor core dumps and shuts down.
The work-around is to not use Constraints Editor to pack a latch into the IOB.
This issue will be fixed in a future software release.