Magento – AJAX cart displaying parent image instead of product image

I have the same problem as described here: Mini-Cart displaying Parent Product Image instead of Product Image. However, I cannot get the accepted solution to work. The problem is that when I add a product to my cart via AJAX the parent product image is shown. Only when I refresh the page is the correct simple product image shown. What am I missing?!?

Layout

app/design/frontend/mymodule/mytheme/layout/mymodule_ajax.xml

<?xml version="1.0"?>
<layout>
    <ajax_checkout_minicart>
        <block type="checkout/cart_sidebar" name="root" output="toHtml" template="checkout/minicart.phtml">
            <action method="addItemRender"><type>simple</type><block>checkout/cart_item_renderer</block><template>checkout/minicart/item/default.phtml</template></action>
            <action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/minicart/item/default.phtml</template></action>
            <action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/minicart/item/default.phtml</template></action>
        </block>
    </ajax_checkout_minicart>
</layout>

Controller

app/code/local/MyModule/Ajax/controllers/CheckoutController.php

class MyModule_Ajax_CheckoutController extends Mage_Core_Controller_Front_Action
{
    public function miniCartAction() {

        $layout = $this->getLayout();
        $update = $layout->getUpdate();
        $update->load('ajax_checkout_minicart'); //load the layout
        $layout->generateXml();
        $layout->generateBlocks();
        $html = $layout->getOutput();
        return $html;
    }
}

Templates

app/design/frontend/mymodule/mytheme/template/checkout/minicart.phtml

<?
$_items = $this->getRecentItems();
$_cartItemsCount = $this->getSummaryCount();
?>
<ul id="minicart">    
    <? foreach ($_items as $item) : ?>
        <?= $this->getItemHtml($item) ?>
    <? endforeach; ?>    
</ul>

app/design/frontend/mymodule/mytheme/template/checkout/minicart/item/default.phtml

<?
$_templateHelper = Mage::helper('templates');
$_item = $this->getItem();
?>
<li>
    <a href="<?= $this->getProductUrl() ?>">
        <img src="<?= $this->getProductThumbnail()->resize(100); ?>">           
    </a>
</li>