Ok, end of chapter 2: im at the catacombs on my way to enter vergen with roch, i reach the final room of the catacombs and dethmold is waiting there along with his commander and some men.

I fought them off, killed 'em all and Deathmold just stands behind the forcefield he created and nothing happens... i searched the whole room, no way out, and i found a "catacomb's key" at a dead body there. Also for some reason i can't loot the commander's body and in the minimap appears as a red dot as if he is still alive. I did the fight 5 times now and always the same result. (?!)

Yeah, I believe it's a bug. Happened to me as well in my first playthrough. Make sure you leave Dethmold for last (kill everyone else first), that resolved it for me.

I've heard of this problem occurring when using finishing moves, but never Heliotrope. I finished the fight using that very skill, so... it's clearly not universal.

Well i've tried not using any group skills and the same happens... only after following mornegroth's advice on leaving Dethmold, in the battlefield, for last did it work. Thanks though guys -)