Call to undefined method Mage_Checkout_OnepageController::isFormkeyValidationOnCheckoutEnabled()

At Magento Onepage Checkout page nothing happens after clicking [Continue] button at Billing information step. It is not possible to proceed to Shipping, Magento checkout just stuck.
The following error logged in PHP error log:

PHP Fatal error:  Call to undefined method Mage_Checkout_OnepageController::isFormkeyValidationOnCheckoutEnabled() in app/code/core/Mage/Checkout/controllers/OnepageController.php on line 374

Cause

SUPEE-9767v2 (and v1 as well) added isFormkeyValidationOnCheckoutEnabled() function into app/code/core/Mage/Core/Controller/Front/Action.php file:

diff --git app/code/core/Mage/Core/Controller/Front/Action.php app/code/core/Mage/Core/Controller/Front/Action.php
index b6eac9e..4f96782 100644
--- app/code/core/Mage/Core/Controller/Front/Action.php
+++ app/code/core/Mage/Core/Controller/Front/Action.php
@@ -188,4 +188,14 @@ class Mage_Core_Controller_Front_Action extends Mage_Core_Controller_Varien_Acti
     {
         return Mage::getStoreConfigFlag(self::XML_CSRF_USE_FLAG_CONFIG_PATH);
     }
+
+    /**
+     * Check if form_key validation enabled on checkout process
+     *
+     * @return bool
+     */
+    protected function isFormkeyValidationOnCheckoutEnabled()
+    {
+        return Mage::getStoreConfigFlag('admin/security/validate_formkey_checkout');
+    }
 }

Solution

As the error says isFormkeyValidationOnCheckoutEnabled() is undefined:

• check if SUPEE-9767v2 was applied properly and the function is defined in app/code/core/Mage/Core/Controller/Front/Action.php:

  $ grep "function isFormkeyValidationOnCheckoutEnabled" app/code/core/Mage/Core/Controller/Front/Action.php
      protected function isFormkeyValidationOnCheckoutEnabled()
  

• Check for local overrides in app/code/local/Mage/Core/Controller/Front/Action.php and app/code/community/Mage/Core/Controller/Front/Action.php:

  $ ls -la app/code/{local,community}/Mage/Core/Controller/Front/Action.php
  

  and apply the changes from patch to local overrides as well
• Disable Magento Compiler

If you have any difficulties with solving this problem or got a similar one, please let us know in comments below, so we can find the solution together.