Error in checkout process

I get the following error on checkout:
[01-Sep-2018 19:07:51 UTC] PHP Warning: sizeof(): Parameter must be an array or an object that implements Countable in /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-enterprise/lib/ext/abstract/class-affiliates-validator.php on line 20
[01-Sep-2018 19:07:51 UTC] PHP Stack trace:
[01-Sep-2018 19:07:51 UTC] PHP 1. {main}() /Applications/MAMP/htdocs/studierenplus/index.php:0
[01-Sep-2018 19:07:51 UTC] PHP 2. require() /Applications/MAMP/htdocs/studierenplus/index.php:17
[01-Sep-2018 19:07:51 UTC] PHP 3. require_once() /Applications/MAMP/htdocs/studierenplus/wp-blog-header.php:19
[01-Sep-2018 19:07:51 UTC] PHP 4. do_action() /Applications/MAMP/htdocs/studierenplus/wp-includes/template-loader.php:12
[01-Sep-2018 19:07:51 UTC] PHP 5. WP_Hook->do_action() /Applications/MAMP/htdocs/studierenplus/wp-includes/plugin.php:453
[01-Sep-2018 19:07:51 UTC] PHP 6. WP_Hook->apply_filters() /Applications/MAMP/htdocs/studierenplus/wp-includes/class-wp-hook.php:310
[01-Sep-2018 19:07:51 UTC] PHP 7. WC_AJAX::do_wc_ajax() /Applications/MAMP/htdocs/studierenplus/wp-includes/class-wp-hook.php:286
[01-Sep-2018 19:07:51 UTC] PHP 8. do_action() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/woocommerce/includes/class-wc-ajax.php:78
[01-Sep-2018 19:07:51 UTC] PHP 9. WP_Hook->do_action() /Applications/MAMP/htdocs/studierenplus/wp-includes/plugin.php:453
[01-Sep-2018 19:07:51 UTC] PHP 10. WP_Hook->apply_filters() /Applications/MAMP/htdocs/studierenplus/wp-includes/class-wp-hook.php:310
[01-Sep-2018 19:07:51 UTC] PHP 11. WC_AJAX::checkout() /Applications/MAMP/htdocs/studierenplus/wp-includes/class-wp-hook.php:286
[01-Sep-2018 19:07:51 UTC] PHP 12. WC_Checkout->process_checkout() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/woocommerce/includes/class-wc-ajax.php:435
[01-Sep-2018 19:07:51 UTC] PHP 13. do_action() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/woocommerce/includes/class-wc-checkout.php:1041
[01-Sep-2018 19:07:51 UTC] PHP 14. WP_Hook->do_action() /Applications/MAMP/htdocs/studierenplus/wp-includes/plugin.php:453
[01-Sep-2018 19:07:51 UTC] PHP 15. WP_Hook->apply_filters() /Applications/MAMP/htdocs/studierenplus/wp-includes/class-wp-hook.php:310
[01-Sep-2018 19:07:51 UTC] PHP 16. Affiliates_WooCommerce_Integration::woocommerce_checkout_order_processed() /Applications/MAMP/htdocs/studierenplus/wp-includes/class-wp-hook.php:288
[01-Sep-2018 19:07:51 UTC] PHP 17. Affiliates_WooCommerce_Integration::process_order() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-woocommerce/lib-2/affiliates-woocommerce.php:2
[01-Sep-2018 19:07:51 UTC] PHP 18. Affiliates_Referral_WordPress->evaluate() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-woocommerce/lib-2/affiliates-woocommerce.php:2
[01-Sep-2018 19:07:51 UTC] PHP 19. Affiliates_Referral_WordPress->add_referrals() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-enterprise/lib/ext/wordpress/class-affiliates-referral-wordpress.php:2
[01-Sep-2018 19:07:51 UTC] PHP 20. Affiliates_Attributes::validate_value() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-enterprise/lib/ext/wordpress/class-affiliates-referral-wordpress.php:13
[01-Sep-2018 19:07:51 UTC] PHP 21. Affiliates_Validator::validate_amount() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-enterprise/lib/ext/abstract/class-affiliates-attributes.php:38
[01-Sep-2018 19:07:51 UTC] PHP 22. sizeof() /Applications/MAMP/htdocs/studierenplus/wp-content/plugins/affiliates-enterprise/lib/ext/abstract/class-affiliates-validator.php:20
maybe
[01-Sep-2018 19:08:13 UTC] payment_method was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, WC_AJAX::checkout, WC_Checkout->process_checkout, do_action(‘woocommerce_checkout_order_processed’), WP_Hook->do_action, WP_Hook->apply_filters, {closure}, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
belongs to it, too?