PHPMailer conflict

Since updating to the latest version of the plugin, I am getting the following error (log detail below). The booking is completed but is not able to send confirmation emails even though email sending is configured through GMail SMTP and not using PHP mail function. The form submission hangs on the front end and users are attempting multiple submits resulting in multiple bookings but no submission or email acknowledgement.

[Fri Mar 24 08:58:30 2017] [error] PHP Fatal error: Cannot declare class PHPMailer, because the name is already in use in /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-includes/class-phpmailer.php on line 28
[Fri Mar 24 08:58:30 2017] [error] PHP Stack trace:
[Fri Mar 24 08:58:30 2017] [error] PHP 1. {main}() /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-admin/admin-ajax.php:0
[Fri Mar 24 08:58:30 2017] [error] PHP 2. do_action() /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-admin/admin-ajax.php:101
[Fri Mar 24 08:58:30 2017] [error] PHP 3. WP_Hook->do_action() /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-includes/plugin.php:453
[Fri Mar 24 08:58:30 2017] [error] PHP 4. WP_Hook->apply_filters() /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-includes/class-wp-hook.php:323
[Fri Mar 24 08:58:30 2017] [error] PHP 5. wpdevart_bc_calendar->wpdevart_form_ajax() /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-includes/class-wp-hook.php:298
[Fri Mar 24 08:58:30 2017] [error] PHP 6. wpdevart_Main->main_form_ajax() /var/www/vhosts/53/284953/webspace/siteapps/WordPress-108332/htdocs/wp-content/plugins/booking-calendar/booking_calendar.p