Photosynth.net hosts both photosynths and panoramas. What you make in ICE, Adobe Photoshop, or Photosynth's mobile app are always panoramas - never photosynths.
The panoramas that Photosynth hosts are meant to wrap around the viewer instead being a large flat panorama. ICE has three camera motion types that it can can understand but the only one that you can upload to Photosynth is one where the camera rotated in place. After you have seen the preview thumbnail in ICE, if you choose to upload to Photosynth or export to HD View (choose HD Zoom Tileset in ICE's format drop down list), then ICE must composite the full resolution spherical panorama, project it onto the inside of a cube, then save each side of the cube for quick loading online.
This takes more memory (both disk space + RAM) than making a flat 2D version of the pano. You'll need to either free up some disk space for ICE or buy more RAM for your computer for ICE to finish the cube map.