Update your configuration.php file to show a more details regarding the error and that should give you some indication of the Plugin that is failing to load.
public $error_reporting = 'maximum'; ... public $debug = true;
From there you can then browse through the #__extensions table of your Joomla instance and find the Plugins with Authentication in the folder field and based on your findings from the messages change the enable field to zero for the offending Plugin.
If the messages don't provide a clear candidate then disable any 3rd party plugins with Authentication in the folder field. The _joomla and _cookie are the only two you probably need to have enabled.
All this will get you able to login, as to how to fix the cause that will come when you find out which plugin is/was failing.