I just faced this error and it was caused by the duplicate static content MIME type in my web.config
Your error was being returned only on the static files - eg images, css, js files were all showing your error.
The way to debug this problem is to look in the web.config under a static content. Here we have the json file extension loaded. This was required on my IIS7 but will kill the app if used on my IIS8 because the json is now pre-loaded at a server level.
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="30.00:00:00" />
<mimeMap fileExtension=".json" mimeType="application/json" />
So solution is to just remove any of these mimeType entries one at the time to confirm which are needed for you and which can kill your app!