Some things to try:
- If using Internet Explorer to view the page, turn friendly HTTP errors off (in Internet Explorer, go to the Tools menu -> Internet Options -> Advanced tab -> untick "Show friendly HTTP error messages" in the Browsing section).
- In the website in IIS 7+, go to "Error Pages" in the IIS section, right-click the "500" entry, go to "Edit Feature Settings", select "Detailed errors" and OK. If "Error Pages" does not show in IIS, install the "HTTP Errors" IIS component.
- Someone was able to fix this problem by installing all .NET Framework updates from Windows Update and restarting the server.
- Check the Event Log -> Application section to see if there are any errors from IIS.
- Check out the first answer here: http://stackoverflow.com/questions/1741439/asp-mvc-in-iis-7-results-in-http-error-403-14-forbidden - it could be that "runAllManagedModulesForAllRequests" is not included in the system.webserver -> modules header thing.