[plt-scheme] problems with MrEd running under Quack emacs mode on WindowsXP

Hi,

I have set up Plt-Scheme on a WindowsXP, with emacs and Quack editor mode,
I followed these steps to set up emacs and quack:
http://bc.tech.coop/scheme/scheme-emacs.htm

My problem is that I am getting fairly strange behavior when I 
run MrEd as the inferior process of emacs (using Quack).

For example, if I evaluate the following code:

***********************************************************
;; Make a frame by instantiating the frame% class
(define frame (instantiate frame% ("Example")))

;; Make a static text message in the frame
(define msg (instantiate message% ("No events so far..." frame)))

;; Make a button in the frame (using keyword-based arguments)
(instantiate button% () (label "Click Me") (parent frame)
             ;; Callback procedure for a button click
             (callback (lambda (button event) 
                           (send msg set-label "Button click"))))

;; Show the frame by calling its show method
(send frame show #t)
*************************************************************

in Quack/Emacs, the window with the correct title shows up, but there is
nothing inside the window. If I hide the window, and give focus back to by right-clicking the tab of the window and tell it to maximize, then the widgets appears - but this time with the correct contents, i.e. with the message "No events so far..." and a button with label "Click Me". The button does not respond to any action. 

Interestingly, everything works fine on this machine when I run the exact same code from the DrShceme IDE.

(some further setup details: instead of what the built in F5 command does, I defined my own MrEd starting procedure by replacing "mzscheme.exe" with
"mred.exe -z" in my c:/home/_emacs file on windows... and it works, i.e. quack and the inferior MrEd do start.)

Any ideas what might be going wrong?

thanks

Rudi