I unfortunately (I guess?) had cleared this cave out previously before receiving this quest line. After getting the quest update to find the cave and completing it I enter the cave and am told to put an end to the skooma operation. However, there are only two people in there compared to the maybe 10 the first time I went in. Even after I kill the two people, the quest does not update. It appears that this is broken and I have no way to complete it. Unfortunate considering this is a key quest in becoming Thane of Riftin and owning a house there...

I have the same problem only it happened to me on the pc