Apologies if this is already covered somewhere, I couldn't find a bug with these keywords.

When you go to view Board Styles, they still have the option to view the demo (http://www.phpbb.com/styles/demo/2.0/index.php?style_id=xxx) - but since those have been deleted (or moved) in the changeover to 3.0, you get an error page instead. It may be time to delete those demo links from 2.0 styles.