Problem Using PHP Code for Images Specific to Post Categories

I’m working on a blog that’s set up to show different image on at the top of each screen depending upon the category. When the blog was upgraded to 3.0.1 we’re now getting an error page instead of the blog or the category. When I take out the code on the category.php file, the blog works again. I’ve disconnected plugins and reconnected… no difference. I’ve taken down the theme and turned on the Default theme. Also no difference. The PHP I’m using begins <?php if (is_category('category name')) { ?> which is the recommended code. It worked fine until the upgrade. Any ideas? Help needed!