Abstract: This short how to explains how to enable better IIS error messages
By default, IIS and therefore ASP.NET hide detailed error information to prevent revealing sensitive information about your web application and shows only an error (e.g. a 500 one). To change that you need to edit the web.conf and add the following values:
<customErrors mode="Off" />
<httpErrors errorMode="Detailed" />
Restart your IIS after that change.
Make sure to change these settings back, once you finished troubleshooting the issue! Otherwise it might revealing sensitive information.