There's a discord for Incarnasoul/Manor here and PC:UE uses the 'Unity Edition' channels here (Salty's Bottled Starlight discord is mostly for other games but PC:UE has some channels there).
I'm not completely sure, but from memory the error can occur as I had a manually added version of Newtonsoft Json and at some point (or via some setting) Unity began including it as well, which conflicted. Moving/deleting the one in the assets folder (Unity/Assets/JsonDotNet) should help.