The template file that IDesign Online can load/create has the ".des" extension. You have to add the ".des" extension to your IIS's serving list (the term is "adding MIME type").

Instruction:

1. For IIS 7.0 or later: download this file web.config and put to your root of server.
2. For IIS 6.0 or earlier: Please ask your hosting company to "add .des MIME type 'application/x-idesign' " for you.