Upgrade task failed when trying to upgrade from Confluence version 2.2.9 to Confluence version 2.8:
Accessing Confluence via browser leads to an error page as shown in screenshot, error.jpg.
It occurs when one of the space in Confluence instance 2.2.9 is using the Left Navigation theme and the theme has been configured via the "Configure theme" session.
A workaround would be to perform an incremental upgrade. For example, first upgrade the Confluence instance from version 2.2.9 to 2.7.x. Confirm the upgrade process is successful and later perform an upgrade again from version 2.7.x to version 2.8.0.