I am using the latest version of Joomla (1.5.22) and K2 (2.4.1). Since yesterday, am getting an invalid token when logging in and out of my site but only in the frontend. I have searched Joomla forum and found this solution from Andrew Eddie.
I've had a look at the login.php file for K2 (mod_k2_login) and it already has <?php echo JHTML::_( 'form.token' ); ?> which most people suggest as the solution.