I don't think that we can say what caused the horizontal band from these two frames an your report that it also appears between frames, alone. You will need to do some further testing: Try to reproduce the same conditions. Shoot some film, use bright lights to illuminate the camera for some of the frames. Shine the lights directly on the camera. Shoot some white and some black stuff. The banding may be easier to figure in one of those instances. Skip a frame or two by leaving the lens cap on with illumination. Then turn the lights off, and shoot the same scenes with no lishts shining on the camera.
After shooting your test roll, try to develop it in EXACTLY the same way. When you scan the film, try to include the space around the frame as well, including the part of the film between frames.
Good luck! Hopefully the problem will just disappear and you will never see it again.