Hi,

If phpBB is installed on a local network, and his "domain" is only a netbios name, cookies will not work.
E.G.: http://intranet/

This is due to the lack of dots in the domain name (like "localhost").

Possible way to fix:
OPEN
includes/session.php

FIND
$domain = (!$config['cookie_domain'] || $config['cookie_domain'] == 'localhost' || $config['cookie_domain'] == '127.0.0.1') ? '' : '; domain=' . $config['cookie_domain'];

REPLACE WITH
$domain = (!$config['cookie_domain'] || $config['cookie_domain'] == 'localhost' || $config['cookie_domain'] == '127.0.0.1' || strpos($config['cookie_domain'], '.') === FALSE) ? '' : '; domain=' . $config['cookie_domain'];