Can't finish "Ruined Garden" quest

[KoRnish]

Hi everyone.

I've stumbled upon a problem (a bug I think) while doing the "Ruined Garden" side-quest. I've killed all the demons in the Garden of Hamaleth, both the random-encounter mobs (got a message confirming it) and all the enemies visible on the map, yet the NPC won't acknowledge it, and won't give my my reward. I know this is only a side-quest, but it is very frustrating, does anybody know any solution for this? I'm playing the polish Steam version.