ariari:
downloaded an extended version
So where and how did you download it? We do not offer any kind of static "extended" version to download.
I suppose, your server just doesn't support "Refresh" header. Disclaimer sets one cookie, identifying its acceptation. In some cases cookie's header may conflict with the other headers sent. That's why I asked you about your server (which I still have no ideas about).
You may try to modify Disclaimer's code in bb_plugins.php and change the line which says
header("Refresh: 0; url=https://www.minibb.com/forums/{$indexphp}action={$action}");
to
header("{$rheader}{$main_url}/{$indexphp}action={$action}");
or directly, to
header("Location:{$main_url}/{$indexphp}action={$action}");
Let me know if it helped.