So my problem is if you try to go to /admin and log-in using an administrator credentials or even the default login set via the _config.php file, it returns the user to the login page without logging them in.
The url says Security/login?BackURL=/admin
I tried flushing cache, rebuilding database, turning dev mode on, etc. Tried manually visiting the admin by typing it in, but it just keeps returning you to the login page.
The only thing I changed recently was to add GD::set_default_quality(100); to the _config.php and even removing it doesn't fix the problem.