My htaccess code above was to ensure that visitors would always use the address from your config file.
Right now, visiting your site, I get this debug message:
GET https://yoursite.top/forum/api/posts?filter%5Buser%5D=2&filter%5Btype%5D=comment&page%5Blimit%5D=20&sort=-createdAt
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>404 Not Found</title>
</head><body>
<h1>Not Found</h1>
<p>The requested URL /forum/api.php was not found on this server.</p>
</body></html>
I managed to register successfully though, so the forum is working.
So, to me this indicate a rewrite error.
I would not be surprised if you have an htaccess file in your root folder that could potentially override something in the forum folder.
Contact me on discord if you want further help 🙂