I recently encountered a rather frustrating issue relating to an ASP.NET 4.5 application that we host using IIS.
Requests for static files were returning with a 500 error with no other information. Attempting to load the file by itself yielded “The page cannot be displayed because an internal server error has occurred.”
I attempted to change settings regarding error detail, to no avail. I couldn’t get anything to return to the client except “The page cannot be displayed because an internal server error has occurred.”
I turned on IIS failed request tracing, and configured the providers. I was finally able to determine that an extra <mimeMap> declaration in our Web.config file was gumming things up. Specifically:
Cannot add duplicate collection entry of type ‘mimeMap’ with unique key attribute ‘fileExtension’ set to ‘.svg’
Because of this extraneous entry, I also was unable to open the MIME Map UI option in the IIS features panel.
Once I removed it, things went back to normal!