CombatAIFix – Fixes AI behavior in combat (for version 3.1).

Is it definitely known that this mod causes crashes, and would a clean version of the game work normally?

No, it's not a clean version; it's with a Heroes mod. Does anyone know how to make them compatible?

Added 3 minutes later

Oh, and please tell those who don't know how to do it! I've been waiting for the mod author to fix this bug.

If you have some modding skills, just add 1 mana to the unit with the active ability; it won't be spent, but it will be required to activate the ability. That's how I solved the problem for myself.