Google announced a change to the officially supported URL for YouTube API ClientLogin requests.
"Previously, developers who wanted to obtain authentication credentials via a username and password had to send their ClientLogin request to https://www.google.com/youtube/accounts/ClientLogin. The new URL that we encourage all developers to use is https://www.google.com/accounts/ClientLogin.
New URL has been functional for a while, and is used by other Google APIs for ClientLogin. Switching YouTube API ClientLogin traffic to standard URL ensures that developers across all of Google's APIs will see standard behavior when logging users in," explains Google.
Details regarding this switch as well as best practices for performing ClientLogin requests can be found in YouTube API documentation. There's one specific change in behavior that's worth calling out: responses from new ClientLogin URL willn't include the YouTube username of the authenticated user, which the old URL's responses did provide.