You can turn on debugging on a single site from the wp-config:
if ($_SERVER['HTTP_HOST']=='site1.example.com')
define('WP_DEBUG', true);
else
define('WP_DEBUG', false);
Solved. The problem was that I used the “WordPress Beta Tester” plugin for upgrading an existing blog to 3.0 alpha. Not a very good idea. Once I deleted the old files and made a svn checkout instead everything worked ok.
