There is a file called "updatescan.rdf" in your profile folder. This file contains the pages that appear in your sidebar, and can be copied from computer to computer. Make sure firefox is not running when you do this!
This means the site could not be downloaded by the scanner. Check that the URL is correct, and if it continues to fail submit a bug.
Also try increasing the scan timeout in the preferences dialog.
Perhaps your character encoding is incorrect. Go to the properties dialog and select "Auto-detect". If this still doesn't work, enter the page's URL in firefox, and look at which character encoding is selected in the "View" menu. Choose this encoding in the update scanner properties dialog.
From the "View" menu select "Toolbars" then "Customise...". Drag the Update Scanner icon from the customise dialog onto the toolbar.