Cannot Login After Drupal 7 migration
First thing to do to see if the login is functioning is use drush uli, which will create a login link for user 1. If that works, after logging in, set your password to test123. Logout, then try to log in with that.

You can also set the password with drush, trying to edit the md5 hash etc is not reliable.

https://drushcommands.com/drush-8x/user/user-password/

This doesn't have anything to do with the files directory.

Another thing to do is check your settings file just to ensure you are talking to the right database. Also check the database to see if the user you are logging in as is not blocked (status).

If you had previously set cookie domain in the settings file, verify that the value is correct.