I saw this a while ago that Google ads are not allowed on error, login, and thank you pages, however, I can't seem to find that in the program policies anymore. did google change them?
It may not be in the policies, but it is in the Terms and Conditions ("https://www.google.com/adsense/terms") under 5.(v) Prohibited Uses
Yes that means whether we can not allow ads on a forums login page that is the index page but most of the forums are having ads on it