I'm using development versions of Emacs and Org mode:

  Emacs  : GNU Emacs 23.1.90.1 (i686-pc-linux-gnu, GTK+ Version 2.18.3)
   of 2010-03-03 on enif
  Package: Org-mode version 6.34trans

When I try to follow (with `C-c C-o') a link to an already
read article in a Gnus nndoc group, I get an error message
like this:

  Couldn't fetch article <000001cabb88$b67f8fc0$237eaf40$@email.ne.jp>

Here is the link location for that message:

  gnus:nndoc+/home/tlm/Mail/RMAIL-201003:RMAIL-201003#000001cabb88$b67f8fc0$237eaf40$@email.ne.jp

I'm able to follow the link if the group is already open
with all old articles visible.

I've attached a patch which is probably not a proper fix.
It solves the problem by making `org-gnus-follow-link'
read the entire group from the start instead of beginning
with one article and trying more if the group fails to open.