Your site will redirect to a generic system error page when some exception happens. To find out the real system error please do following.

If you can TS to your server, you could open up event viewer and check the "DotShoppingCart" eventlog.

If you don't have console access to your server, there is another way. Turn off custom errors in your web.config file and you will see the error from the web page.

<customErrors mode="Off" defaultRedirect="GenericErrorPage.htm">
            <error statusCode="403" redirect="NoAccess.htm"/>
            <error statusCode="404" redirect="FileNotFound.htm"/>
</customErrors>