Error on checkout & mail

Hi,

Since two days my mail is doing (for me) weird things.
When I have new orders it didn’t get an e-mail, and that problem solved automatically.

But now when my customers check out, only the amount * price is visible in the checkout page and later in the e-mail. So there is no product visible at the checkout page.

When I turn PHP errors on i get this:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘kia_woocommerce_order_item_name’ not found or invalid function name in /customers/7/b/c/rent-fun.be/httpd.www/v2/wp-includes/class-wp-hook.php on line 300

How can I solve this?
Thank you so much!