Have you deleted any properties?
I'm pretty sure I haven't.
I also went back and checked a few images that I thought could be the problem and double checked their properties compared to the original Aero and they all seem to match up.
The only property that consistently appeared different (that I didn't change myself) between the two themes was the ATLASRECT:RECT property.
Edit:
I went back and found that under the Reflections section I had added a property and removed the main reflection image. If I try to import it again the program crashes.
Update:
Alright, I finally managed to get the image imported and now the issue is gone. Thanks for suggesting checking over my properties, otherwise I may not have found the problematic image and property.
Thanks again!