phpMyAdmin was complaining about "Invalid hostname for server 1. Please review your configuration."
The error is in /var/lib/phpmyadmin/config.inc.php and seems to be a security bug in Lenny.
$cfg['Servers'][$i]['host']=''; phpinfo();//'] = 'localhost';
After saving your file, no error occurs anymore
However, this error is due to a security leak in phpMyAdmin. Upgrade your Debian and you should be fine. However, just to be sure, you can do the following:
sudo nano /usr/share/phpmyadmin/.htaccess
Deny from all
Save and check whether http://www.yourdomain.nl/phpmyadmin/config.inc.php gives a 403-error. If it does, you're good