With the upgrade. Lots of things in the php.ini if not changed are different default to 4.1.1
Now.
Most likely they are errors of things like this
vars
Variables that are undefined. from Say a Url.
If you have mysite.com?site=site
and you are using $site change that to $_GET['site'];
Use of Isset and Sometimes if needed empty will be required
If this doesnt help.
Exactly what kinda errors are you getting. As this is the main difference that i can remember, but that was from 4.2 to 4.3