I've also encountered this bug, however when I've reloaded the save it no longer occurred. The enemy isn't using a mana requiring weapon, however I was trying to train mana growth so I may have tried to use one when the soldier didn't have enough mana.
Edit - I think it happens when one of your soldiers tries to use an ability without having enough mana, and if a different soldier then successfully uses an ability it resolves it.
So to replicate have a soldier use a mana required ability and then move someone.
Edit 2 - It has to be an innate ability, not a staff or glove ect... but something like the ignite ability.