I've done a bit more investigation.
The error page that opens in a new window also provides the following link:http://arduino.cc/blog/2011/01/18/forum_migration_jan_24/
suggesting the problem is part of the migration.
I am logged in to the the website when I click on the link (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1285962838
) from the webpage http://arduino.cc/en/Main/ArduinoBoardUno
However the the window that opens up, to the old forum, shows me as not logged in. I log in and it takes me to the home page of the old forum(not the page that I needed). I close the window and go back to the original page that I came from (http://arduino.cc/en/Main/ArduinoBoardUno
I then click on the link (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1285962838
) again and it goes, correctly, to the relevant post.
Problem with old and new cookies?
(O/S Ubuntu 11.10, Browser Firefox 12.0)