500 Internal Service Error in codeigniter on sub domain

I had an admin panel written in codeigniter on my website root folder say example.com. Then I needed to move it to a sub directory named admin so i can upload my main website on the root folder. the admin panel was now on example.com/admin/. Then i created a subdomain for my website named admin.example.com and pointed it to my sub directory named admin. But when i tried to visit admin.example.com it gives a 500 internal server error. I thought it was due to .htaccess but my .htaccess file has this code:

<IfModule authz_core_module>
    Require all denied
</IfModule>
<IfModule !authz_core_module>
    Deny from all
</IfModule>

Now i don’t know whats causing the problem ? The original working was that when someone visited the example.com he was redirected to example.com/auth/ and same happens when someone visits the admin.example.com he is redirected to admin.example.com/auth/login. I even tried to change the base path in codeigniter config.php to $config['base_url'] = 'http://admin.example.com/'; but it is not working. So please let me know if someone can help. The only solution i found was that move the whole admin panel back to root folder but it is now good as i have to host a static website there. I am waiting for your answers.