Admin folder in subfolder not working...

@HarrysR,

I think that your config file is wrong.

I think that....
$config['base_url'] = 'https://sub.domain.com/admin/';
should be:
$config['base_url'] = 'https://sub.domain.com';

My understanding of subdomains is that when calling the subdomain you use...
sub.domain.com and domain.com/admin not sub.domain.com/admin

I have a subdomain on one of my accounts and the url links listed above (minus the last one) are the ones I use and they work perfectly. When I type sub.domain.com/admin I get an 404 error message too.