Google+ login not working on Appointments+

Setting login required to yes in Appointments+ and choosing only Google+ brings up the login button on the product page. The user is asked to login with their Google profile in a separate window, asks permissions then window disappears and screen refreshes. No login details are recorded and the user is asked to login again. The user is unable to continue to book an appointment.

I checked previous issues and found this where the developer suggests it is to do with the server needing to be configured to allow URL's in GET query requests (although this is the Events plugin) -

I contacted my host and they have whitelisted some modsecurity rules conflicting with the domain in the server. This has not solved the issue.

There are no API settings in the Appointments+ login required section for Google+, only Facebook and Twitter (which we do not require). Is this function now out of date and not supported by Google? I've seen that other social login plugins have a lot of settings that are required to use Google to login.