Fatal Error & Service Unavailable

We have an intermittent, but regularly occurring, issue with a site where a WooCommerce transaction will fail to complete and the order success screen will not be shown, but payment will be taken. If the place order is subsequently clicked then the order goes through.

Looking in the error log at the time this happens I found the following

[18-Jun-2020 15:48:20 UTC] PHP Fatal error: Uncaught Error: Call to undefined function is_ajax() in /home/account/mysite.com/wp-content/plugins/woocommerce-multi-currency/frontend/location.php:67
Stack trace:
#0 /home/account/mysite.com/wp-includes/class-wp-hook.php(287): WOOMULTI_CURRENCY_Frontend_Location->init(”)
#1 /home/account/mysite.com/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#2 /home/account/mysite.com/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#3 /home/account/mysite.com/wp-settings.php(540): do_action(‘init’)
#4 /home/account/mysite.com/wp-config.php(83): require_once(‘/home/account/…’)
#5 /home/account/mysite.com/wp-load.php(37): require_once(‘/home/account/…’)
#6 /home/account/mysite.com/wp-blog-header.php(13): require_once(‘/home/account/…’)
#7 /home/account/mysite.com/index.php(17): require(‘/home/account/…’)
#8 {main}
thrown in /home/account/mysite.com/wp-content/plugins/woocommerce-multi-currency/frontend/location.php on line 67
[18-Jun-2020 15:48:21 UTC] PHP Fatal error: Uncaught Error: Call to undefined function is_ajax() in /home/account/mysite.com/wp-content/plugins/woocommerce-multi-currency/frontend/location.php:67
Stack trace:
#0 /home/account/mysite.com/wp-includes/class-wp-hook.php(287): WOOMULTI_CURRENCY_Frontend_Location->init(”)
#1 /home/account/mysite.com/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#2 /home/account/mysite.com/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#3 /home/account/mysite.com/wp-settings.php(540): do_action(‘init’)
#4 /home/account/mysite.com/wp-config.php(83): require_once(‘/home/account/…’)
#5 /home/account/mysite.com/wp-load.php(37): require_once(‘/home/account/…’)
#6 /home/account/mysite.com/wp-blog-header.php(13): require_once(‘/home/account/…’)
#7 /home/account/mysite.com/index.php(17): require(‘/home/account/…’)
#8 {main}
thrown in /home/account/mysite.com/wp-content/plugins/woocommerce-multi-currency/frontend/location.php on line 67

This is how the error presents at the front-end: https://bit.ly/30VkMI4

At the back end the first order that does process, but for which no success screen is shown, does not show the currency conversion in red, but the order that does succeed (when ‘Place Order’ is clicked again) does show the currency conversion, see here: https://bit.ly/2NfIpmB