my projects has files in branches so most of the files live in Attic
the link to file diff is not working from hudson:

https://myserver/cgi-bin/viewcvs.cgi/project/hibernate/
HibernateClientDao.java.diff?r1=1.1.4.11&r2=1.1.4.11.2.1

working link:

https://myserver/cgi-bin/viewcvs.cgi/project/hibernate/Attic/
HibernateClientDao.java?r1=1.1.4.11&r2=1.1.4.11.2.1

two notable differences:
no suffix ".diff" and additional path "Attic"

suffix ".diff" is optional in ViewCVS
if you don;t add ".diff" it knows how to redirect to Attic.