500 Error in C# ASP.NET Application

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!

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