CI 2.2.6 subfolder route 403 error

To be honest I dont know exactly, I'm not a apache config (.htaccess) guru. You will have to do some research, trial and error etc.

It's easy to have /admin/ redirected to /index.php so you can use it as a controller name (even if the folde exists) but that will also set /admin/css/somefile.css to use that same controller and try to call a method called 'css' etc.
You could make a css,js,img method in the admin controller to read the file inside the admin folder but to serve those kind of files through php for this reason is just bad practise and horrible performance wise.

The easiest thing to do I guess would be to just rename the admin folder (something in the line of assets, or seperate those files into /js/admin/file.js, /css/admin/other.css etc) or use a different controller name like 'administrator', 'cms' etc).