Apollo303

I`ve got the exact same problem, the quest is to follow the arms dealer. Unfortunatly, as in the above screenshot, you follow the AD into that cellar behind the house and a fight errupts..... after the fight the quest marker points outside, I go outside and the marker jumps to just inside the south opening near the crop fields, but there is noone there, there is no cave/cellar, nothing highlights I.e trees, rocks or even the ground. I think this might be bugged or something, is there a way to finish this quest ?!?