After updating to WHMCS 8.13, a Smarty: Default handler: No template default content error is displayed when an admin clicks  Login as Owner on the Client's Summary tab in the Admin Area:

--> Smarty: Default handler: No template default content for 'file:templates/childtemplate/includes/captcha.tpl' <--

In WHMCS instances with a Child Theme and Captcha Form Protection enabled for Login Forms, the system does not properly fall-back to the parent theme if the captcha.tpl template file is not present in the child theme.

The issue has been logged with our development team with case reference WHMCS-22229.

Copy /templates/twenty-one/includes/captcha.tpl file to the /templates/$childtemplate/includes/ directory.

Where $childtemplate is the name of your custom theme.