Hi @la_vet,
Can you describe how the system behaves when they attempt to use it?
One likely culprit is the session_check_ip option in config.inc.php. If this is On, change it to Off. If users are behind a proxy pool, such as a national firewall, then their IP address may appear to change during their session. If session_check_ip is On, OJS will consider the session invalid when this happens.
Regards,
Alec Smecher
Public Knowledge Project Team