Hi @rvdb,
If you see this intermittently (only for some users), I suspect you may need to disable the session_check_ip option in config.inc.php. See this thread for more information.
OJS 2.3.1-2 is pretty old – it was released in 2009 – and I would definitely recommend upgrading in any case.
Regards,
Alec Smecher
Public Knowledge Project Team