How to configure a copy of Codeigniter website on a subdomain?

Hi,
imho, first thing to check, if you set your subdomain as test.domain.com, try to access... test.domain.com without the www which means: comment those two lines
<code>
RewriteCond %{HTTP_HOST}(.*) !^www\. [NC]
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R,L]
</code>
in your htaccess file with an #

second thing i would suggest if this subdomain is a test one, but you may already have done this, create a dedicated db for the subdomain else whatever you change in the db will appear on the production domain too...

hope it helps
have a nice day