I had the same issue. What happened for me though was that I opened up two incompatible replays (which you must do by opening one replay and then loading the other one in the editor), and then I focused the camera on the second, incompatible one I loaded. To fix this, save the camera track as a clip, leave the editor, start by opening up the replay that you want to lock on to, add the other car, and import the camera clip.