since it will display the page if you enter a page name, but show an error if there is no page name, there is a problem in the default document settings.
The Default documents that IIS is aware of are:
if no document is specified in the URL it will try to load one of those files.
The error you get suggests that either one of the default files doesnt exist or there is an error in one of them.
In reality, this is not a problem with WebAssist software. There really isnt really problem at all since you can load files in the browser by specifying a file name.