Not staying logged in.

Users are having trouble staying logged in on the forum since the upgrade to PHP 5.3 from PHP 5.2.10. None of the cookie settings changed, as everything worked on 5.2.10 - here are the values of what is set, session wise:

Authentication: Db
Session Length: 36000
Allow persistent logins: Yes
Persistent login key expiration length (in days): 3
Session IP validation: A.B
Validate browser: Yes

I'm also using PHP 5.3's new mysqlnd, instead of libmysql. And for $dbms in config.php, it's set to mysqli