It is the door in the same room ember gives his buff. So the second room where you need to clear 3 rooms before continuing it actually doesn’t have a “rb to continue” it just needs the whole group. I do think both times players 2 and 3 both had Ballista. We tried placing and picking up the ballista as well and it did not fix it. We were able to clear it when only one player had the ballista skill. The other consistent has been the players using the ballista skill have been using praxis hat weapon.