gnus-summary-post-news error in 21.5 "corn"

[Guido Van Hoecke]

(emacs-version)"XEmacs 21.5  (beta21) \"corn\" (+CVS-20050705) [Lucid]
(i586-pc-win32, Mule) of Wed Jul 06 2005 on WODAN"

(gnus-version)"Gnus v5.10.7"

When I start gnus, select one of my nnfolders, enter 'a' or select
'Post, -> Send a message (mail or news)' from the summary buffer, I
get an empty buffer and following backtrace:

Debugger entered--Lisp error: (invalid-operation "Selecting deleted or non-existent buffer")
  set-buffer("*Article*")
  (save-excursion (set-buffer gnus-article-buffer) (message-fetch-field "to"))
  (concat user-full-name " <" (save-excursion (set-buffer gnus-article-buffer) (message-fetch-field "to")) ">")
  eval((concat user-full-name " <" (save-excursion (set-buffer gnus-article-buffer) (message-fetch-field "to")) ">"))
  gnus-configure-posting-styles("nnfolder:xemacs-beta")
  #<compiled-function nil "...(7)" [gnus-setup-message-group gnus-article-copy nil gnus-configure-posting-styles] 2>()
  run-hooks(#<compiled-function nil "...(7)" [gnus-setup-message-group gnus-article-copy nil gnus-configure-posting-styles] 2>)
  apply(run-hooks #<compiled-function nil "...(7)" [gnus-setup-message-group gnus-article-copy nil gnus-configure-posting-styles] 2>)
  run-mode-hooks(message-mode-hook)
  message-mode()
  message-pop-to-buffer("*mail to xemacs-beta at xemacs.org*")
  message-mail("xemacs-beta at xemacs.org")
  gnus-post-news(post "nnfolder:xemacs-beta")
  gnus-summary-post-news(nil)
  call-interactively(gnus-summary-post-news)
  (dispatch-event "[internal]")

This only happens the first time. I can avoid the problem by starting
with an 'f', dismissing that buffer, and then selecting 'a'.

Needless to say that I do not experience this with 21.4.17, using the
same gnus.