Now I'm thinking in the possibility of some error in vegetation or something like that.
In the CTD against the slaves, any combination of units of army 1 attacking them causes CTD. I've made the rebel army to move from its position by attacking with a larger army 2, that didn't pursuit them in the strategic map. Next turn I attacked with the very same army 1 and the battle was fought without problems.

This seems to indicate that the problem comes from the position of the armies, not from the composition of them.