Magento 2 – How to Fix Checkout Infinite Loop

The error was from Temando Shipping SaveCheckoutFieldsObserver.
You can try fixing this by overriding Temando's SaveCheckoutFieldsObserver

In your di.xml file, add this:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Temando\Shipping\Observer\SaveCheckoutFieldsObserver" type="Vendor\Module\Observer\Temando\SaveCheckoutFieldsObserver" />
</config>

Then create observer file at:

app/code/Vendor/Module/Observer/Temando/SaveCheckoutFieldObserver

<?php

namespace Vendor\Module\Observer\Temando;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Exception\NoSuchEntityException;
use Temando\Shipping\Api\Data\Checkout\AddressInterface;
use Temando\Shipping\Api\Data\Checkout\AddressInterfaceFactory;
use Temando\Shipping\Model\ResourceModel\Repository\AddressRepositoryInterface;

 /**
 * Save checkout fields with quote shipping address.
 *
 */
class SaveCheckoutFieldsObserver extends \Temando\Shipping\Observer\SaveCheckoutFieldsObserver 
{
    /**
     * @var AddressRepositoryInterface
     */
    private $addressRepository;
    /**
     * @var AddressInterfaceFactory
     */
    private $addressFactory;
    /**
     * SaveCheckoutFieldsObserver constructor.
     * @param AddressRepositoryInterface $addressRepository
     * @param AddressInterfaceFactory $addressFactory
     */
    public function __construct(
        AddressRepositoryInterface $addressRepository,
        AddressInterfaceFactory $addressFactory
    ) {
        $this->addressRepository = $addressRepository;
        $this->addressFactory = $addressFactory;
    }
    /**
     * @param Observer $observer
     * @return void
     */
    public function execute(Observer $observer)
    {
        /** @var \Magento\Quote\Api\Data\AddressInterface|\Magento\Quote\Model\Quote\Address $quoteAddress */
        $quoteAddress = $observer->getData('quote_address');
        if ($quoteAddress->getAddressType() !== \Magento\Quote\Model\Quote\Address::ADDRESS_TYPE_SHIPPING) {
            return;
        }
        if (!$quoteAddress->getExtensionAttributes()) {
            return;
        }
        // persist checkout fields
        try {
            $checkoutAddress = $this->addressRepository->getByQuoteAddressId($quoteAddress->getId());
        } catch (NoSuchEntityException $e) {
            $checkoutAddress = $this->addressFactory->create(['data' => [
                AddressInterface::SHIPPING_ADDRESS_ID => $quoteAddress->getId(),
            ]]);
        }
        $extensionAttributes = $quoteAddress->getExtensionAttributes();
        if(!is_array($extensionAttributes)){
            $checkoutAddress->setServiceSelection(array());
        }else{
            $checkoutAddress->setServiceSelection($extensionAttributes->getCheckoutFields());
        }

        $this->addressRepository->save($checkoutAddress);
    }
}

Create custom class & copy required functionality of vendor/magento/module-checkout/model/Session.php class file of Magento 2.3.3 version in it.

Then, use that custom class in Magento 2.3.4 version where you have added dependency of Magento\Checkout\Model\Session class file.

Or you can use '\Magento\Checkout\Model\Cart' class file to get all visible items of cart.

$this->cart->getCheckoutSession()->getQuote()->getAllVisibleItems();