How to fix this ngon? I have Mesh Check Addon which says that I have a bad polygon (red vertex). How can I fix it? When I triangulate all faces and set Shading: smooth,  black shadows appear on the model.
Maybe just switch to wireframe mode "Z", box-select the general are (don't grab anything useful), then merge it at center with ctrl-m. Afterwards use snap to vertex on the individual axis, to make sure it ends up back in place.