Why am I getting error 500.0 in when using IIS Express (significant debug info included)

You may have some mime code in the Web.Config file like this:

<mimeMap fileExtension=".svg" mimeType="image/svg+xml" />

if so, you should remove mimeMap before adding like this:

.
..
...
....
  </system.web>
  <system.webServer>
    <staticContent>
      <remove fileExtension=".svg" />
      <mimeMap fileExtension=".svg" mimeType="image/svg+xml" />
    </staticContent>
  </system.webServer>

....
...
..
.

This may be related to a conflict between the IIS Express applicationhost.config and your web.config.

A mimeType was added to my local web.config that was already present in the applicationhost.config and IIS Express started serving many 500 errors.

You may also notice this error in your Windows Event Log, "The directory specified for caching compressed content is invalid. Static compression is being disabled."

More info: http://blog.degree.no/2013/04/the-directory-specified-for-caching-compressed-content-is-invalid-static-compression-is-being-disabled/

I removed the mimeTypes from the web.config and the issue was resolved.

applicationhost.config location: C:\Users\[User]\Documents\IISExpress\config

reinstalling iis express appears to have resolved this problem

Are you using any PLINQ(.AsParallel) or Parallel.For or similar methods? I've found that the AggregateExceptions they throw aren't handled well by MVC (In my case, I got a blank 500 page and nothing in Logs/Event logs).

I identified the problem by paying attention to the "First Chance" exceptions which are logged in the Debug output window in VS. Try running the site, waiting for it to error, clearing the Debug window and the reloading the page. Do you see anything useful?

There might be a problem with your config files, specifically web.config and applicationhost.config.

The only way I know how to diagnose this, is to publish a build to regular IIS, and in IIS Manager check the "Error Pages" module of the Site. If it is indeed an issue with those config files, you'll get a popup here with something along the lines: "Cannot add duplicate entry of type …". Google accordingly.
If the "duplicate entry" is of type "error", you can try adding the following element below httpErrors:

<remove statusCode="404"/>

That is, assuming you have an Error element below the HttpErrors with statusCode 404. (I had once.)

In my case the issue was caused by using windows authentication + firefox + wrong domain login with insufficient permissions.

Details: My dev machine is using windows domain "domA", the application will run in an environment accepting logins from domain "domA" and "domB". While on the webserver both will work, on my dev machine only my account in "domA" has sufficient permissions to access all files of the application. In IE "domA" is used by default while in FF I get a login dialog where I did login with my account from "domB".

Confusing issue I know, but maybe there is someone else having the same situation. At least it will help me the next time I run into it. :)

Just in case catches someone else out, I had similar error but the issue was I had updated my web.config and there was an extra ">" character where it should be.. so malformed web.config file can cause the same behaviour.