The mod_rewrite rules changed since you last installed this plugin. Unfortunately, you must remove the old supercache rules before the new ones are updated. Refresh this page when you have edited your .htaccess file. If you wish to manually upgrade, change the following line: %1$s so it looks like this: %2$s The only changes are "HTTP_COOKIE" becomes "HTTP:Cookie" and "wordpressuser" becomes "wordpress". This is a WordPress 2.5 change but it&#8217;s backwards compatible with older versions if you&#8217;re brave enough to use them.