0::logout.php:50: session_destroy() [<a href='function.session-destroy'>function.session-destroy</a>]: Session object destruction failed

each time one logs out.

This is because session_destroy() is attempted after a session_unset() in access/logout.php - the "@" notwithstanding. If this is required for some installations, then the following can replace the last 2 statements in it:

session_unset();
if (session_id() != '') session_destroy();

For PHP 5.4+, use the following:

session_unset();
if (session_status() == PHP_SESSION_ACTIVE) session_destroy();