Okay, you are using a very old version of PHP (4.0.6). Look in your ad_network.php file, and replace all instances of _SERVER with HTTP_SERVER_VARS, clear your ad_network_ads.txt file, and try to validate.
BTW, I updated the default ad_network.php file so that if $_SERVER isn't set (because it's an old version of PHP), it sets it to $HTTP_SERVER_VARS. That way the users of old PHP systems don't need to worry about it on an update.