Even when they still have movement point left sometimes units will not move, you can select them and give them a new destination but they will not move. Everything else still works.
Saving and reloading the save will allow the units to move again.
I have now seen it twice, both times i ordered an army to move to and attack an enemy army in my territory. However i did not have sufficient movement points so i ended up next to the enemy army. During the turn my intended victim army was destroyed by the city, after that none of my army/units would move.
Sending a save game would be pointless since a load of said game save fixes the problem.
