Troubleshooting User Authentication

Long story short, I have a website with a securityassist user login system and a user level conditional display throughout that works perfectly. I've duplicated the site for use elsewhere, as the new site needs the exact same user login and user level protections.

I've gone back and double checked that I've duplicated and uploaded all support folders, scripts, etc. The database structure was duplicated as well (though not the data). But on the new duplicated site, I can't get the user login system working. I've even gone to completely wiping and rebuilding the login system with the "Create Security Pages". When I login with an account created with the registration page (I've confirmed it wrote to the DB properly), the page reports that I'm logged in, but subsequent pages that have conditional areas with 'rules' show as if I'm not logged in.

I thought I'd be able to duplicate the old site and hit the ground running, but I've been pulling my hair out for many hours.

How do I troubleshoot something like this? I'm unsure where to even start.