Are you aware of the move to HTTPS. detailed in the Yahoo Developer Network blog (below)? http://yahoodevelopers.tumblr.com/post/75633763287/yahoo-contacts-and-profile-apis-move-to-https
Yahoo has enabled HTTPS access to Yahoo Contacts and Profile APIs and now requires all developers using the Yahoo Contacts APIs and Profile APIs to use the HTTPS protocol (and port 443). The domain name social.yahooapis.com is the same. Since last month, Yahoo is limiting all access to Yahoo’s Contacts APIs and Profile APIs to secure SSL connections only. If you haven't made the switch to HTTPS, your users will not be able to access their Yahoo Contacts / Profile data via your service.