This problem arose after upgrading to Magento 18.104.22.168. which is a patch release that patches a number of security flaws in Magento. After installation of the patch some parts of the front-end template stayed blank. Review of the exception.log file shows a number of exceptions that seem to occur from the template filter classes of Magento.
The error is:
exception 'Exception' with message 'Notice: Undefined variable: block in /data/web/public/app/code/core/Mage/Core/Model/Email/Template/Filter.php on line 187' in /data/web/public/app/code/core/Mage/Core/functions.php:245