Thank you for your message.
The most common reason this happens is a conflict with another plugin, usually something offering functionality related to security or the login page.
If they interfere with WordPress’ normal login features, they can prevent Google Apps Login from being allowed a chance to interpret the code/session parameters, which is why nothing happens.
It may not be obvious why this suddenly changed with the upgrade to WordPress 4.0, but it may just be that WordPress suddenly started calling the plugins in a different order after they had been reactivated following the upgrade.
You could try deactivating other plugins to see if Google Apps Login starts to work again, in which case at least we know the culprit and can start to work out how to get around it.
Please could you send me an email with any further details, especially of the plugins you are using, and if deactivating any caused it to work again? This would be useful for others, even if you can get it to work now without help.
Email [email protected]
Thanks,
Dan