JSP pages don't work with database after postgres downgrade

I recently had to downgrade a version of postgres on a devleopment box
from 7.3.2 to 7.2.2 to bring it in line with a production server. I
reinstalled postgres, postgres-server and postgres-jdbc. Now when I
try to run pages from a web application written in JSP using
apache/tomcat they don't work.

By "don't work" I specifically mean that the 1st page is one where the
user's username and password are checked against a table in the
database. This is coming back with a logon failed message even though
I know that the username and password are correct. So I am assuming
that the JSP pages are having trouble connecting to the database.

Does anyone know how I can go about debugging this to find the root of
the problem?

Regards,
Phill