Hey, my site gets validated fine when I do it, but if I do it a second time a few minutes later it gets an error until I do it again. I was just notified this morning that my ads were taken down because the links could not be found. However, when I checked, the ads were there, and I went and validated and it worked right away. Are there errors with the validation system or something? I would feel much better not having to wonder if the system is going to correctly review my site. At least we get an email though telling us the ads are down? That is definately a plus!
What's the url that you got via email? Did you recently update your php code without flushing out the ad_network_ads.txt?
It uses the google api to spot check pages. I did a quick check and google has the following page listed: http://www.net-tracking.net/links/links31.htm And there are no co-op ads on it.