I've removed the log_sender plugin, which was causing that error in the logs, but logging in from that first page still gives me the same result.
These are the plugins we have currently installed:
$config['plugins'] = array('http_authentication','squirrelmail_usercopy', 'userinfo', 'new_user_dialog', 'rcs_mobile_switch', 'rcs_mobile_options', 'filters');