Have you tried a force-attack? To perform a force-attack click the sword icon near the "formations" button, then click a peaceful target. Hey, i just tried and IT WORKS ! Thank you. Still not sure if it is a bug or a feature
I have the exact same issue, adra dragon cannot be attacked, it stays green, as do xaurips, but it is still able to attack my party after i moved towards the treasure, no way to fight back. This is a serious bug obviously. Btw, tested with lastest 1.04 beta patch on Steam.