Just found your question here because it happened to me as well. Here’s what I did and hopefully it will fix / help find the problem.
- Deleted all .suo
- Terminate IIS Express
- Close VS2013 and re-open
- Open your project and try to start it (F5)
- You probably have an error on your web.config file. For me it was an
<appSettings>being there twice. - Fixed the web.config file, save and boom, error is gone.
Hope it will also fix your situation.