I actually got my WordPress 2.0 working now. I had this problem but I’d done some modifications to the wp-settings.php file.
Where it says:
if ( !defined(‘COOKIEPATH’) )
define(‘COOKIEPATH’, preg_replace(‘|https?://[^/]+|i’, ”, get_settings(‘home’) . ‘/’ ) );
if ( !defined(‘SITECOOKIEPATH’) )
define(‘SITECOOKIEPATH’, preg_replace(‘|https?://[^/]+|i’, ”, get_settings(‘siteurl’) . ‘/’ ) );
I had these lines commented which resulted in the same redirect problem with the / turning to %2Fs. I uncommented it and I could login to the website again. Check your wp-settings.php file that these lines are there.