When they where only two tiles left (of the same kind) lying on top of each
other, I clicked "shuffle". A dialog popped up, telling me that it couldn't
find a playable configuration. When I clicked on Ok, nothing happened. Only
closing the window with the windowmanager close button helped. 

To reproduce: Play until a few stones are left but make sure that at least
two stones are still above each other. Then shuffle until you can remove
everything but those two stones. Now click on "shuffle" again and the
faulty dialog will pop up. The dialog also shouldn't be called "Error" IMO,
because that sounds a little bit harsh for such a logical problem.