AJAX Quick Checkout Error Message

Hello,

i installed the Extansion "AJAX Quick Checkout" and installed it.
But now i become following error message in my cart, after i go to Checkout page:

Call to undefined function Opencart\System\Helper\Utf8\strlen(): in /mnt/web509/d1/08/561208208/htdocs/extension/ajax_quick_checkout/catalog/controller/ajax_quick_checkout/cart.php on line 451

The extansion:
https://www.opencart.com/index.php?rout ... n_id=15580

Opencart Version: 4.0.2.3

Have anyone a idea what the error message means and how i can fix it?

Thanks for help,
Greeetings from Germany

This code is on line 451:

'value' => (\opencart\system\helper\utf8\strlen($value) > 20 ? \opencart\system\helper\utf8\substr($value, 0, 20) . '..' : $value)

i have no more ideas whats wrong

Thank you for help!

Is there a reason why you won't contact the extension author on this?