When exporting the release build of your AIR application using Adobe Flex Builder if you get an Adobe AIR Error 303, it implies that the problem is with your icons. Most probably the problem might be due to pointing the icon to the wrong folder.

src is the root folder. Suppose your icon is in a folder called assets inside the src folder, then inside the app descriptor, the path should be like:

<icon>
    <image16x16>/assets/Icon-16x16.png</image16x16>
    <image32x32>/assets/Icon-32x32.png</image32x32>
    <image48x48>/assets/Icon-48x48.png</image48x48>
    <image128x128>/assets/Icon-128x128.png</image128x128>
</icon>

If you don't have say a 16px icon then remove that node instead of leaving it blank.