I have the same issue. Tested on a clean setup running only WordPress 5.9.3, WooCommerce 6.4.1, MailPoet 3.88.2 and Add-on WooCommerce MailPoet 3 version 1.1.8.
Error message:
[05-May-2022 09:34:24 UTC] PHP Fatal error: Uncaught TypeError: Argument 3 passed to MailPoet\API\MP\v1\Subscribers::subscribeToLists() must be of the type array, bool given, called in /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/mailpoet/lib/API/MP/v1/API.php on line 66 and defined in /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/mailpoet/lib/API/MP/v1/Subscribers.php:82
Stack trace:
#0 /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/mailpoet/lib/API/MP/v1/API.php(66): MailPoet\API\MP\v1\Subscribers->subscribeToLists('636', Array, true)
#1 /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-place-order.php(108): MailPoet\API\MP\v1\API->subscribeToLists('636', Array, true)
#2 /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-place-order.php(37): MPWA_Place_Order::save_subscriber_record(Array, Array)
#3 /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/add-on-woocommerce-mailpoet/add-on-wooc in /customers/2/e/6/jawbreaker.se/httpd.www/wp-content/plugins/mailpoet/lib/API/MP/v1/Subscribers.php on line 82
The user is shown the error message “Internal Server Error” and cannot proceed until newsletter checkbox is unchecked.