Appointments + conflict

I have appointments+ plugin with email notifications. Those notifications are not coming. I have WP Main SMTP plugin for my emails sending and I have set it up to use Gmail API with oauth (as opposed to SMTP).

When I set it to SMTP with login/password, it works, but if I send it to Gmail oauth, it throws an error:

2018/02/02 22:10:47 [error] 2046#2046: *45404 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Call to undefined method Google_Client::getClassConfig() in /wp-content/plugins/appointments/includes/external/google/Logger/Abstract.php:126
Stack trace:
#0 wp-content/plugins/appointments/includes/gcal/class-app-gcal-logger.php(6): Google_Logger_Abstract->__construct(Object(Google_Client))
#1 /wp-content/plugins/appointments/includes/gcal/class-app-gcal-api-manager.php(58): Appointments_Google_Calendar_Logger->__construct(Object(Google
_Client))
#2 /wp-content/plugins/appointments/includes/gcal/class-app-gcal-api-manager.php(43): Appointments_Google_Calendar_API_Manager->get_client()
#3 wp-content/plugins/appointments/includes/class-app-gcal.php(71): Appointments_Google_Calendar_API_Manager->__construct()
#4 /wp-content/plugin" while reading response header from upstream, client: 71.92.61.85, server: applianceg****com, request: "POST /wp-admin/adm
in-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:", host: "applianceg***.com", referrer: "https://applianceg***.com/schedule-appointment/"

the combination of Appointments+ with Gmail oauth does not work, but I still can send test emails and forgot password emails