Some days ago the Magento security update SUPEE-8788 was released. This update fixes a number of critical vulnerabilities. To fix an existing shop one could either apply the SUPEE-patch or upgrade the shop to Magento 1.9.3. However, after updating I experienced a little issue when trying to reach the shop again.
A PHP Exception popped up:
Notice: Undefined index: session_expire_timestamp in /data/web/public/app/code/core/Mage/Core/Model/Session/Abstract/Varien.php on line 461
Even after flushing the cache this problem appears. The undefined index and path of the file that raises the exception gives away that this problem might have to with Magento’s session handling. I started with deleting any existing session cookies in my browser. This made the error message disappear. Be aware that this is not an appropriate solution to this error!