First thing I’d do is test the DOF. Is it working? Then I wouldn’t worry about the error message.
After that, is the empty parented to a head control bone? Does that bone actually exist, or did it perhaps get deleted somehow (like baking to deform bones, or renaming)? What is “camera.free.turtle.002” object? What is Camera.010 (don’t get confused with Camera.008)? I might check “focus”'s relations (and the relations of its relations), its parentage and its constraint targets, for anything weird.