The error is "500 - Internal server error." The log shows it primarily as 500 0 0 218, but there are also some instances of 500 0 0 202.
What happens is this: if I simply make an unedited copy of the Flex Viewer index.html file and name it index.asp and then use that index.asp in the URL, I get the error. In contrast, if I create a new directory under wwwroot and inside it I place an index.asp file with some generic html, it works fine with or without any asp code inside.
I can get around this by creating a separate entry asp page and put html code in it to redirect to the index.html, but it seems unnecessary.