OnePageCheckout Doesn’t Place Order After Update in Magento 1.9

I've updated my Magento environment from 1.7.0.2 to 1.9.2.3. As usual, I stumbled across a few problems. Most of those problems I'm able to fix after a while, but I've discovered a very strange problem that I just can't seem to wrap my head around.

I'm using OnePageCheckout for logged in users to pay their products (OneStepCheckout works fine). I can go through every step, but when I push the "Submit Order" button, nothing happens. I don't have any JavaScript or PHP errors.

At first I thought my form needed a form key, since that's necessary for newer Magento versions. So I added this line of code:

review = new Review('<?php echo $this->getUrl('checkout/onepage/saveOrder', array('form_key' => Mage::getSingleton('core/session')->getFormKey())) ?>', '<?php echo $this->getUrl('checkout/onepage/success') ?>', $('checkout-agreements'));

This didn't fix it.

I tried to echo the saveOrder URL with it's form key and check what happened when I went directly to that URL. The link told me that nothing happened because I didn't check the Terms of Agreements box (which I did).
So obviously I wanted to know what would happen when I turned the terms of agreements off in my Magento backend.
It gave me the following error:

Fatal error: Call to undefined method Mage_Customer_Helper_Data::getFullCustomerName() in /home/users/tetesftp/xxxxx/app/code/core/Mage/Checkout/Helper/Data.php on line 238

I'm stuck right now. Where do I look? Usually it's a template problem as well, but now all the other templates (like RWD) aren't working either.

I hope someone can help me with this! Thanks in advance!