Several dungeons require to have killed every mob in order to proceed through a door to the next section.
Converting monsters with the Paladin skill “Conversion” turns them friendly permanently, thus unkillable by the party. Converted monsters do not count as dead. This makes the doors unopenable, since the monster is still alive.
I’ve experienced this in the lv115 dungeon, but I assume it’s the same way in any other instance that requires to kill all the monsters.
Logging out and exiting the dungeon doesn’t solve the problem, the effect persists even if the Paladin has left the group.
I suppose that any converted monster should be counted as dead for proceeding further in instances.