Simple question first: do you have your Web Server enabled (Tools >> Options >> Web Server Configuration >> Enable Web Server)? What is the exact URL you are given? What is your Server's root directory (Tools >> Options >> Web Server Configuration >> Root Directory)? Are you able to host any other pages from the Web Server? Was the Web Server working before? What browser are you using? Do you get the same error when you copy and paste the URL into the web browser? If you browse to the .htm file (Root Directory\VI Name.htm) and click on it, are you able to see the VI (this will only open it locally, not through the Web Server)?
Robert Mortensen
Software Engineer
National Instruments