Fixed!
I added this in wp-config.php, and now it works!
define('COOKIEPATH', '/');
reference:
https://code.google.com/p/googleappengine/issues/detail?id=11354#c7
Thank you!
Sorry, my colleague did certain change in wordpress flies. It seems to not be a problem of plugin.
Since I clean installed WP and this plugin, it worked fine.
