For the last 10 months I have an intermittent problem with a woocommerce shop. A few times a week a customer will report getting the Wordpress error message during checkout,"The site is experiencing technical difficulties". They are unable to checkout and the sale is lost.

I'm unable to trigger the error even when duplicating the order.

I have debug running and at the time of the last failed order this was added to the debug log

[25-Nov-2019 04:46:31 UTC] PHP Notice: get_woocommerce_term_meta is deprecated since version 3.6! Use get_term_meta instead. in /home/aawcomau/public_html/wp-includes/functions.php on line 4435 
[25-Nov-2019 04:46:39 UTC] PHP Warning: array_keys() expects parameter 1 to be array, null given in /home/aawcomau/public_html/wp-content/plugins/woocommerce/includes/wc-cart-functions.php on line 468 
[25-Nov-2019 04:46:39 UTC] PHP Warning: array_keys() expects parameter 1 to be array, null given in /home/aawcomau/public_html/wp-content/plugins/woocommerce/includes/wc-cart-functions.php on line 439 
[25-Nov-2019 04:46:39 UTC] PHP Warning: current() expects parameter 1 to be array, null given in /home/aawcomau/public_html/wp-content/plugins/woocommerce/includes/wc-cart-functions.php on line 440 
[25-Nov-2019 04:46:39 UTC] PHP Fatal error: Uncaught TypeError: Argument 2 passed to Flexible_Shipping_Plugin::woocommerce_default_shipment_method() must be of the type array, null given, called in /home/aawcomau/public_html/wp-includes/class-wp-hook.php on line 286 and defined in /home/aawcomau/public_html/wp-content/plugins/flexible-shipping/classes/class-flexible-shipping-plugin.php:393 Stack trace: #0 /home/aawcomau/public_html/wp-includes/class-wp-hook.php(286): Flexible_Shipping_Plugin->woocommerce_default_shipment_method(NULL, NULL, false) #1 /home/aawcomau/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(NULL, Array) #2 /home/aawcomau/public_html/wp-content/plugins/woocommerce/includes/wc-cart-functions.php(453): apply_filters('woocommerce_shi...', NULL, NULL, false) #3 /home/aawcomau/public_html/wp-content/plugins/woocommerce/includes/wc-cart-functions.php(417): wc_get_default_shipping_method_for_package(0, Array, false) #4 /home/aawcomau/public_html/wp-content/plugins/woocommerce/includes/class-wc-cart.php(1281): wc_get_chosen_ in /home/aawcomau/public_html/wp-content/plugins/flexible-shipping/classes/class-flexible-shipping-plugin.php on line 393 
[25-Nov-2019 04:46:53 UTC] PHP Notice: get_woocommerce_term_meta is deprecated since version 3.6! Use get_term_meta instead. in /home/aawcomau/public_html/wp-includes/functions.php on line 4435 

While the plugin ‘Flexible Shipping’ is mentioned, the same error was being triggered before, when I was using Table rate shipping instead of Flexible Shipping.

Being unable to create the error makes it impossible to fault find.

Is there any clue in the error message code that can help me fix this problem?