I'm getting this error sometimes from app\code\core\Mage\Core\Model\Session\Abstract\Varien.php . I'don't know when it's happening just pops up sometimes.
Check the permissions of that folder where PHP and your Magento stores session files if you have access to this server. The user that PHP is running as must be able to read and write to that folder.
Also check if there are any other scripts that delete session files. For example there is a separate cron job in Debian that cleans up old session files. Now - if something's wrong with the clock or timezones that "being an old file" may occur in wrong time. However, the latter is just my guess.