Magento Checkout show blank page after SUPEE-10570 security patch

Problem description

Proceeding to Magento Checkout page when logged in as a customer results in a blank page after SUPEE-10570 or Magento upgrade with Magento Compiler enabled. There are no any PHP errors logged or shown unlike PHP Parse error: syntax error, unexpected ‘[‘ PHP incompatibility case with SUPEE-10752 patch.

Cause

The problem occurs on trying to load a customer object using any available Magento methods, be it via Mage::getModel('customer/customer') or Mage::getSingleton ('customer/session') at getPasswordTimestamp() function. It is caused by Zend_Date class loaded twice when compilation is enabled, one time from from includes/src/__checkout.php file and the second one from includes/src/Zend_Date.php.
Logging of this error is disabled by suppression @ operator on the line supposed to load this Zend class, otherwise it would print clearly the following error to PHP error log:

Fatal error: Cannot redeclare class Zend_Date in includes/src/__checkout.php on line 50796 

Solution

Disable Magento Compiler. Nowadays any PHP Opcache outperforms it anyway.

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.

Posted in: Fatal errors