The variable

in /template/login_forum.html is not filled with a value when seeing login mask wanting to log into a password protected forum.
To fix that
find in /includes/functions.php

$template->assign_vars(array(
'S_LOGIN_ACTION' => build_url(array('f')),
'S_HIDDEN_FIELDS' => build_hidden_fields(array('f' => $forum_data['forum_id'])))
);

change it to
$template->assign_vars(array(
'S_LOGIN_ACTION' => build_url(array('f')),
'S_HIDDEN_FIELDS' => build_hidden_fields(array('f' => $forum_data['forum_id'])),
'FORUM_NAME' => $forum_data['forum_name'],
));