There seems to be a persistent bug with the site authentication tool in the Google AdSense publisher controls.

Some publishers are complaining about errors being returned when trying to use the feature. There is a Google AdSense Help thread that says the issue has been happening for about a week now. Google said it was fixed, but some publishers are still complaining about it not working. One posted the error:

Yesterday, AdSensePro Jennifer, an official Google representative, said, “Should be fixed now. Let me know if it’s still not working.” But a few publishers came in afterwards to inform her that it is not working.

This site authentication feature launched in July 2007, but has been seen since June and early May of 2007.

Forum discussion at Google AdSense Help.