Who Sees Ads Errors

I get the following errors on my site with the plugin Who Sees Ads for WPMU that I downloaded from here:

Warning: Invalid argument supplied for foreach() in /home/blogg/public_html/wp-content/mu-plugins/wp_ozh_whoseesads.php on line 461

Warning: array_keys() [function.array-keys]: The first argument should be an array in /home/blogg/public_html/wp-content/mu-plugins/wp_ozh_whoseesads.php on line 855

Warning: Invalid argument supplied for foreach() in /home/blogg/public_html/wp-content/mu-plugins/wp_ozh_whoseesads.php on line 855

I only get the errors on one site. All other sites work fine.

Anyone have an idea why this happens? Could it be caused by some adcode that the plugin doesn't really handle that well?