find code into file: register.inc.php
change from
$lang['new_user_email_txt'] = str_replace("[activate_link]", $settings['forum_address'] ."index.php?mode=register&id=".$new_user_id."&key=".$activate_code, $lang['new_user_email_txt']);

to:

$lang['new_user_email_txt'] = str_replace("[activate_link]","http://". $_SERVER['SERVER_NAME'] ."/index.php?mode=register&id=".$new_user_id."&key=".$activate_code, $lang['new_user_email_txt']);

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