I'm not sure this is specific to this chain quest but rather to the terrain type which was not properly excluded:

I was doing "Defeat Waerloga the Dragonlord". The third step of this quest "The Black Gate: Find a Darkling Shaman" poped a quest location right on an impassable terrain as shown below.

This terrain is quite peculiar though, these are the impassable rivers found in the Frozen domains of Vertrar (Elemental Lord of cold/frost I defeated in a previous quest). Maybe you forgot to exclude these tiles from the possible quest locations.