Make sure your browser is configured to show errors, like in IE:
- Turn off the friendly error message option in the browser as follows:
- In Internet Explorer, on the Tools menu, click Internet Options.
- On the Advanced tab, under the Browsing section, click to clear the Show friendly HTTP error messages check box, and then click OK.
- Close the browser.
Turn on detailed errors in IIS.
- Internet Information Services (IIS) Manager —> Your Web Site —> Error Pages -> Edit Feature Settings -> select "Detailed errors"
Make sure your web.config does not have any nodes like:
<httpErrors>
<error statusCode="500" path="/somedir/500.asp" responseMode="ExecuteURL" />
</httpErrors>
and
<customErrors mode="RemoteOnly" defaultRedirect="~/404error.aspx" />