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.