I am playing through the Fallen Enchantress scenario. (I have not tried sandbox yet.) In chapter 10 tactical combats, certain characters turn comes up, his picture is displayed on the bottom left, but he has no icon on the tactical map and therefor can not move. (Having the computer finish combat works.) I restarted my computer and can reproduce this bug. It is not always the same character. It is usually the first turn.
Work around: remove a couple units from your party BEFORE starting tactical combat. (I remove some of the soldiers I 'rescued' earlier.) I guess I have too many units in my party?