Magento2 - Success page redirecting to cart after checkout

Question

I'm having an issue where an Order is placed, it is created but the user is redirected to the cart page, with an empty cart and an error message saying "There was a error when showing the success page.".

I'm not getting any errors in var/log, also not in my php or nginx/error log. I'm using onepagecheckout with Klarna (KCO) and Magento 2.3.5.

I've noticed that if I comment out

$session->clearQuote();

on Success.php

The success page renders properly.

Why is the

$session->clearQuote();

used in Success.php,

Can I just extend Success.php and remove the

$session->clearQuote();

? Or will that break something else?