Exact same issue, ADF-based web scan produces blank pages in file saved to computer, and yes we tried both sides of document, 2-sided, just to make sure correct side faces the scanner. Our OS X is 10.8.x, so it's a problem beyond Snow Leopard. Document size is not an issue, as a single page scan is being attempted.
A glass-fed web scan works fine, so this is not a network issue, but a problem when a web scan on OS X uses the ADF from the browser interface (in this case, Firefox). The glass-fed scan previews the image and saves the file 100% correctly.