Well, after some hard work investigating external auth problem on not only google+ one, but also on Facebook and Twitter.
I've done intensive tests, tried all of them, and adjusted configuration on developer page of each provider.
Now all should be okay,
if you have any authentication problem, logoff from your browser application auth provider and the try to login again.
example if you want to log with facebook
go to www.facebook.com
to see if auto logged into, if so then click log/off
now with the same browser windows (or new tab) go to https://community.hallard.me
and log in and choose Facebook authentication it will ask you login/password and then log into the forum
If you can't log on the forum drop me a PM