[plt-scheme] why does the ellipsis not work?

The following is extracted and simplified as much as possible from a bigger piece of code.
Sorry that I could not condence it any further.

; file lazy.ss ----------------------------------------
(module lazy mzscheme
 (provide lazy)
 (define module-name (gensym 'module-name-lazy-))
 
 (define-syntax lazy
  (syntax-rules ()
   ((lazy expr) (lazy/proc 'expr))))
 
 (define (lazy/proc expr)
  (let/cc cc
   (eval
    `(module ,module-name 
     "lazy-language.ss"
     ,cc ,expr))))
 
 ; inorder to make require superfluous
  (lazy/proc 'initialize)
  (eval `(require ',module-name)))
  ; module name quoted for 3.99.0.9

; file lazy-language.ss ---------------------------------
(module lazy-language mzscheme
 (provide macro scheme
  (rename mod-begin #%module-begin)
  (rename #%datum #%top)) ; allow free vars
 
 (define-syntax (macro stx) 
  (syntax-case stx ()
   ((_ (id keywords rule ...) ... expr)
    #'(letrec-syntaxes
     (((id ...) (#%app values (syntax-rules keywords rule ...) ...)))
     expr))))
 
 (define-syntax scheme
  (syntax-rules ()
   ((_ x) (#%app eval 'x))
   ((_ x y ...) (#%app (#%app eval 'x) y ...))))

 (define-syntax mod-begin
  (syntax-rules ()
   ((_ cont x)
    (#%module-begin
     (#%app (#%datum . cont) x))))))

; using the above -------------------------
; definitions window
(require "lazy.ss" )

(lazy
 (macro (a () ((a x y) (scheme list x y)))
  (a p q))) ; --> (p q) ok

(lazy
 (macro (a () ((a x ...) (scheme list x ...)))
  (a p q))) ; --> (p q) ok

(lazy
 (macro (a () ((a x ...) (scheme list x ...)))
  (a p q r))) ; --> error: a: bad syntax in: (a p q r)

Run: interactions-window:
Welcome to DrScheme, version 3.99.0.9-svn6jan2008 [3m].
Language: Textual (MzScheme, includes R5RS)
(p q)
(p q)
lazy.ss:12:3: a: bad syntax in: (a p q r)
> 

This used to work in all earlier versions of MzScheme. Now many things have changed, I know. What change do I overlook? It seems like in the calls to syntax lazy, the ellipses are taken for a common identifier. Any help will be very much appreciated. I already tried to parse the rules given to syntax macro, replacing every occurrence of identifier '...' by a locally introduced ellipsis, but that did not help.
Jos Koot
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