Please enable and see the detailed errors of the web messages, as this is a very general message without giving much information on the details for security reasons.
With the detailed error message, you will be able to locate the real issue.
Please run your browser on the server, you will find details of the error, as your server recognizes that you are a local and will show it to you.
You can read the log of your server by using the Event Viewer, you will also see the details of the error.
On IIS 6
On IIS 7
<httpErrors errorMode="Detailed" />
You can avoid the Debug=true. You only need to close your custom errors for a while and you will see the detailed error page.