When trying to log into the backend after entering the username and password and as soon as you click on login, I get the following error. Fatal error: Call to undefined method: stdClass->onAuthenticate() in /home/sitename/libraries/joomla/user/authentication.php on line 121 (Joomla! 1.5)

Solution

Change the file permissions of the plugin folder to at least 755. And try again and the backend should load perfectly