Had this problem a while ago on a MU site that I have – Can’t remember exactly what I needed to do to solve the problem, but it involved adding a few lines to the wp-config.php file – trouble is, looking at the file I cannot remember which lines I added, although I have a feeling that it is the below. For obvious reasons I have taken the unique phrases out, but you can visit the URL listed to generate them. Hope it helps –
/**#@+
* Authentication Unique Keys.
*
* Change these to different unique phrases!
* You can generate these using the {@link http://api.wordpress.org/secret-key/1.1/wpmu/salt WordPress.org secret-key service}
* You can change these at any point in time to invalidate all existing cookies. This will force all users to have to log in again.
*
* @since 2.6.0
*/
define('AUTH_KEY', '');
define('SECURE_AUTH_KEY', '');
define('LOGGED_IN_KEY', '');
define('NONCE_KEY', '');
define('AUTH_SALT', '');
define('SECURE_AUTH_SALT', '');
define('LOGGED_IN_SALT', '');
/**#@-*/
Thanks buddy, but after adding the Authentication Keys to my wp-config.php file, I am still getting the same error.
Adam
Did you try deleting all cookies frist? May be a problem with the one that is already there, that was created without the keys?
Also, found my post from a few months back, so may hopefully be of use to you.
http://wordpress.org/support/topic/cookie-error-site-not-letting-me-log-in?replies=7
