In earlier versions of IIS, error messages from classic ASP scripts were sent to a Web browser, by default. Because these error messages might reveal sensitive information to malicious users, IIS 7.0 disables this feature by default. When your classic ASP scripts encounter an error in IIS 7.0, you receive the following error message by default:

An error occurred on the server when processing the URL. Please contact the system administrator.

If you are the system administrator please click here to find out more about this error.

Workaround 1:

If you are using the IIS Manager navigate to the ASP icon on the right pane.  

Drill down to Debugging Properties and update the Send Errors To Browser parameter to True

Workaround 2:

Upload a web.config file with the following text to the root of your application.

<xml version="1.0" encoding="UTF-8">
<configuration>
    <system.webServer>
        <asp scriptErrorSentToBrowser="true" />
    </system.webServer>
</configuration>

If  the root of the application already contains a web.config file then all you need to do is insert the bold code in the <configuration> tag of the existing web.config.