In this quest, Manthir is phased out of the house he is supposed to be in. The marker is there, but the NPC himself is not. This basically makes the quest impossible to complete. Neither abandoning, nor reloading helps. After communication with the Support, I was suggested to report this as a bug. I have attached the screenshot for that.
P.S. Same area, around Greenheart - there is a random Summoner there chanting for Daedra in the wild, bit she appears to be looped and no other activity follows - she stays there forever.