I think IIS (web.config) settings are overwritten and enforced before actually EPiServer can handle that.

If you really need custom 500 error page for rest of the world and for admins - show detailed view of the error, you could create a bit more complex 500 error page and behind the scene check permissions for the current user - if he/she is able to see detailed messages.

using EPiServer.Security

..

if (PrincipalInfo.Current.IsPermitted(Permission.DetailedErrorMessage))

If user is permitted you can use exception manager to render detailed error page:

ServiceLocator.Current.GetInstance<IExceptionManager>().RenderHttpRuntimeError(exception);

Ohhhh Great!!!!

Thanks a lot!! 

And how should I get the error message in the controller action? :)

I think you have to check Server.GetLastError() or similar.

Yeah, in the meantime I've found that but it is null.

I found this post: 

http://sunali.com/en/2008/01/16/why-does-server-getlasterror-return-null/

I've added globalErrorHandling="Off" in episerver.config but it still gives me null. I am not sure if I should try with Context.AllErrors because the post is from 2008 :)

investigate AllErrors collection..

I tried with it but it is also null. I think that Episerver handles that error and I have to find out how. 

Thanks anyway :)

Try to go upwards from EPiServer.Global.OnError event handler :)

At the end, this is how I done it:

  private void Application_Error(object sender, EventArgs e)
        {
            // Code that runs when an unhandled error occurs

            var exc = Server.GetLastError();
            if (exc != null)
            {
                if (PrincipalInfo.Current.IsPermitted(SystemPermissions.DetailedErrorMessage))
                {
                    ServiceLocator.Current.GetInstance<IExceptionManager>().RenderHttpRuntimeError(exc);
                }
                else
                {
                    var httpException = exc as HttpException ?? new HttpException(500, StringConstants.InternalServerError, exc);
                    Response.Clear();
                    switch (httpException.GetHttpCode())
                    {
                        case 403:
                            Response.Redirect("~/403.html", true);
                            break;
                        case 404:
                            Response.Redirect("~/404.html", true);
                            break;
                        default:
                            Response.Redirect("~/500.html", true);
                            break;
                    }
                    Server.ClearError();
                }
            }

        }

I removed the code from the web config.

I am not sure if it is the best solution but it works for now. If anybody has some better solution, please share :)

Thanks Valdis :)