What happens is if the association with Google hasn't been created, Google asks for the authentication etc. Then after a few redirects to the URL below with title "Connecting..." (there doesn't seem to be anything "private" in the URL). The same happens if I first authenticate the application on a desktop browser and then try and login on the phone. The browser is logged in on Google (I can access Gmail etc.), but XenForo authentication fails.
This is from trying to login here at Xenforo.com:
Edit: I also see a "Welcome back" from G+ at the bottom when I visit the forums on a mobile browser.
Login also fails on Windows Phone 8.1. However the accounts.google.com page does close on WP8.1, but nothing happens on the XenForo page. I.e. the pulldown login menu stays open, and if I reload the page I'm still not logged in.
Signing in with Twitter seems to work as expected on WP7.8