I just got this error and it was caused by a duplicate static content MIME type in the web.config

This error was being returned only on static files - eg images, css, js files were all saying this error (text by itself, no other html or text in the response).

The way to debug this is to look in web config under static content. Here we have a json file extension loaded. This was required on IIS7 but will kill the app if used on IIS8 because json is now pre-loaded at the server level.

    <staticContent>
        <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="30.00:00:00" />
        <mimeMap fileExtension=".json" mimeType="application/json" />
    </staticContent>

So solution is to remove any of these mimeType entries one at a time to confirm which are needed and which kill your app!

Update

Actually the best solution was provided by a commenter here. You can remove and then add, which will always work regardless of whether it is already defined or not. Like this:

<remove fileExtension=".json" /> 
<mimeMap fileExtension=".json" mimeType="application/json" />

I think the best first approach is to make sure to turn on detailed error messages via your web.config file, like this:

<configuration>
    <system.webServer>
        <httpErrors errorMode="Detailed"></httpErrors>
    </system.webServer>
</configuration>

After doing this, you should get a more detailed error message from the server.

In my particular case, the more detailed error pointed out that my <defaultDocument> section of the web.config file was not allowed at the folder level where I'd placed my web.config. It said

This configuration section cannot be used at this path. This happens when the section is locked at a parent level. Locking is either by default (overrideModeDefault="Deny"), or set explicitly by a location tag with overrideMode="Deny" or the legacy allowOverride="false". "

Modify your web.config to display the server error details:

<system.web>
  <customErrors mode="Off" />
</system.web>

You may also need to remove/comment out the follow httpErrors section

  <system.webServer>
    <httpErrors errorMode="Custom">
      <remove statusCode="404" />
      <error statusCode="404" path="/Error/Error404" responseMode="ExecuteURL" />
      <remove statusCode="500" />
      <error statusCode="500" path="/Error/Error500" responseMode="ExecuteURL" />
      <remove statusCode="403" />
      <error statusCode="403" path="/Error/Error403" responseMode="ExecuteURL" />
    </httpErrors>
  </system.webServer>

From my experience if you directly have a server error, this may be caused from an assembly version mismatch.

Check what is declared in the web.config and the actual ddl in the bin folder's project.