I checked back through the old messages (using the Search link above) and "light cone" in the text field. It appears that light cone fall-off doesn't work in Carrara, and that light cones interfere with other processes, like g-buffers (post-render effects). So light cones may also interfere with transparency. Maybe your cube isn't set to cast shadows, but I doubt that would change the effect, especially if light cones also interfere with that property. Is your cube set for shadows?

Even though some post-render effects are disallowed if the output file format isn't correct, I doubt that using an actual tiff render (rather than the area render) would give you the correct light cone effects. Did you try a tiff render in the render room (with other effects disabled)?