I received the following error meesage while trying to log in to the admin interface: JAuthentication: :authenticate: Failed to load plugin: plgauthenticationjoomla Not sure of the details but try restoring your plugins directory from a backup. This resolved this error message for me and I could log in. For other suggestions, see: http://forum.joomla.org/viewtopic.php?f=624&t=667525&view=previous