Drupal 7: logout not working

How can I best debug this?

I have two Drupal 7 sites on the same server, with different modules, but with almost the same configuration. On one site the logout is working correctly, but on the other site, logout does not work. If you click on the logout link, the user isn’t logged out and the sessions stays open, even after you restart the browser. The only thing that did work was deleting browser history (cookies).

For now, I temporarily “fixed” this issue by setting

ini_set(‘session.cookie_lifetime’, 0);

in settings.php so that the user is logged out after he or she closes the browser, but this “solution” is not what I prefer of course.

Note that I tuned both php.ini and Apache for improved caching and improved security.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/22/feed