How to set web.config file to show full error message

This can also help you by showing full details of the error on a client's browser.

<system.web>
    <customErrors mode="Off"/>
</system.web>
<system.webServer>
    <httpErrors errorMode="Detailed" />
</system.webServer>

If you're using ASP.NET MVC you might also need to remove the HandleErrorAttribute from the Global.asax.cs file:

public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
    filters.Add(new HandleErrorAttribute());
}

Not sure if it'll work in your scenario, but try adding the following to your web.config under <system.web>:

<system.web>
    <customErrors mode="Off" />
    ...
</system.web>

works in my instance.

also see:

CustomErrors mode="Off"

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