i don't have any experience with tunnel fly-through's, but the fact that the un-rendered area is perfectly vertical. not even slightly diagonal looks unlike geometry to me. if you render out an image sequence of this, find a bogus frame, then go back to your sequencer in carrara, set the time to frames, you should be able to get to the exact frame that you're looking for. then try to test render just that frame to see what happens, and or look for objects in the way. might also be usefull to figure out where in the tunnel this occurs and render the animation from a camera outside the tunnel to see if anything comes flying through the tunnel wall. good luck.