I have the same problem.

The quest "Ritual of the berserker" is gone from my quest list.
It is not in completed quests either.

I found the nightshade and i think there was a notification about the quest at that point, but that is hours ago. Now I tried to figure out how to return the nightshade that is sitting in my inventory, but there is no quest for it...

I have the Nightshade in my inventory, and i looked up a questguide online that told me what the next step should be, so I went to "the Gallows Tree" but Bjorn is not reacting and the quest is just missing.