PHP errors in your plugin kept me from saving settings on the Woocommerce General Settings page.
Please make sure all of your clean/sanitize scripts are running on the correct variable types.
wp-content/plugins/woocommerce-product-addon/inc/functions.php:112
strtolower() expects parameter 1 to be string, array given
Fix:
$field_name = (is_string($field_value)) ? $domain . ' - ' . sanitize_key($field_value) : $domain . ' - ' . $field_value;
wp-content/plugins/woocommerce-product-addon/inc/functions.php:119
stripslashes() expects parameter 1 to be string, array given
Fix:
$field_value = (is_string($field_value)) ? stripslashes($field_value) : $field_value;