This message occurs because the site has effectively timed out due to the size/complexity of your file (or rather, the PDF viewer has timed out, and the site has locked itself out/gotten into a loop and gotten lost inside itself). The main culprit is usually images that are way above the recommended resolution (we only need 300 DPI images) or Photoshop PDF files with extra layers or lots of effects that can be merged down. You don’t need to flatten the files; just remove any unused layers and merge down your effects.