Problem with login

Hello,
I have a problem with your plugin: Sometimes, the login process fails and return a 503 error on _process_loggin.php.
I’m sure the user is logged in on facebook.
I activated the debug log over the plugin settings, but no email with logs is sent when the error occurs.
I use your pugin on a big site with more than 200000 users.
Have you got any advice to resolve or debug this problem?

Thank you in advance.

http://ww.wp.xz.cn/plugins/wp-fb-autoconnect/