Amasty Customer Attributes Extension conflicts with Magento Paypal Express Payments by overriding the html block output on the Order Review page. Here is a quick fix.

handleBlockOutput.php

<?php

// Edit your Amasty Custoemr Attributes
// Observer.php file

// find the handleBlockOutput function

    public function handleBlockOutput($observer) 
    {
        /* @var $block Mage_Core_Block_Abstract */
        $block = $observer->getBlock();

        if ($block instanceof Mage_Checkout_Block_Onepage_Billing) {
            return false;
        }

        if ($block instanceof Mage_Checkout_Block_Onepage_Shipping) {
            return false;
        }

        if ($block instanceof Mage_Paypal_Block_Standard_Form) {
            return false;
        }
        
        // rest of the function continues here....
        // ...
        // ...
        // ...

?>