Hey !
I am having the exact same problem with Paypal, when order is paid and is about to be redirected I get :
PHP Fatal error: Uncaught --> Smarty: Unable to find template function 'renderLogo' <--
thrown in /var/www/xxx/shop/vendor/smarty/smarty/libs/sysplugins/smarty_internal_runtime_tplfunction.php on line 50
I changed theme/classic/templates/checkout/checkout.tpl but it did not change anything, same error (fresh cache, fresh browser).
Does anyone have an idea ? Since I'm on 1.7.8.3 no more customer can pay with Paypal