Hi again,
Out of curiousity I searched and found a pdf version of the Contax G2 manual. There is a small window on the back cover to show the type of film used. It may well be that some light manages to leak onto the film, except that in that case you would think that an overall fog rather than a line would occur. I also note that the camera has an in-betwen frames data imprinting function, perhaps a malfunction there may be the cause. In any event this is not a user-fixable problem. Hopefully the camera is still under warranty.
Jean-Michel