I came to te Buddhism SE site and clicked a vote button as a guest which prompts the user to sign up or log in.
I selected "sign in using Google" as I do on most other SE sites but I received a message from Google's OpenID system:
400 (BAD REQUEST) That’s an error.
OpenID auth request contains an unregistered domain: https://buddhism.stackexchange.com/users/authenticate/
That’s all we know.
I was able to successfully link my Google OpenID to the site via this Meta site, but this sounds like an issue with the OpenID registration for this SE site (and possibly others.)