My great concern is that this is a carrara issue. I have had other transparency problems. Usually at an angle. Several people have had this problem. I just checked the file and even made a new one...it was fine and a new one didn't fix it. I tried many options. However, the realistic sky might actually be a problem because that has also come up with this and I do have that set.I will go check. I used alpha and the transparency method. However, I didn't do a layers list. This is very annoying.

No need to think outside the box....
    Just make it invisible.