With Internet Explorer 8 and 9, after entering the Web Player URL, it briefly shows the login page and then displays, "The Webpage cannot be found" error or a blank screen. In some cases, this will happen after you login and the Spotfire Library is not displayed.
The "More Information" on this error says this: "This error (HTTP 404 Not Found) means that Internet Explorer was able to connect to the website, but the page you wanted was not found. It's possible that the webpage is temporarily unavailable. Alternatively, the website might have changed or removed the webpage."