First one idea: Wouldn't it be better if your saved a spritesheet (a png and an xml file) instead of multiple images. I know its harder to achieve but a lot better in terms of future use and performance. In fact this was an idea that I'm considering. Sam Rivello suggested it some months ago.
The first time the app is started means that it's using the Dynamic Atlas right? You get the error when loading the files then.
If the files are being generated you might want to check the way you are saving/loading them.
I'll try to take a look at your code, but in the meantime try to load that image as a BitmapData and add it to the regular Display List as a Bitmap, just to make sure that there are no issues with the image generation.