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:
We apologize for the inconvenience, but we are unable to process your request at this time. Our engineers have been notified of this problem and will work to resolve it
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.
Forum discussion at Google AdSense Help.