It has been reported already -
http://atwar-game.com/forum/topic.php?topic_id=8339
All the units die with 0 losses because the units are not supposed to fight the second battle and so the system breaks. There are several variations of the problem with no solution seen, but the characteristic signs are stack combination near a city, and the stack fights a battle where the stack is combined, and again in the city it is moved into.