wp-config.php not affecting my site

Question

I just received a backup of my WP site from the developer that includes the WP files and the database .sql file. I'm trying to install it on my server, but my WP build is ignoring wp-config.php. When I access domain.com/wp-admin I get Internal Server Error:

Steps I've taken:

1. I uploaded the ZIP file with all WP files to the server and unzipped. Everything is now on the server, including /wp-admin, /wp-content, /wp-includes and so on...
2. I created a new database with names that match the DB_NAME and DB_USER from wp-config.php below (except for the prefix).
3. I uploaded the .sql file to the new database. Database is now populated.
4. I updated wp-config.php so the database fields match my database. I had to update the prefix xxxx because it has to start with my hosting service's username. I kept the rest the same, including the password.

define( 'DB_NAME', 'xxxx_dbname' );       // Only updated prefix xxxx
define( 'DB_USER', 'xxxx_dbuser' );       // Only updated prefix xxxx
define( 'DB_PASSWORD', '[userPassword]' ); // I set the DB user password to match this one
define( 'DB_HOST', 'localhost' );
define( 'DB_CHARSET', 'utf8' );
define( 'DB_COLLATE', '' );
$table_prefix = 'gw48ep_';    // This matches all tables on DB

I also reset all the 'AUTH_KEY's and salts to 'put your unique phrase here'.

When I visit my site, the content displays fine. But when I visit domain.com/wp-admin or /wp-login.php, I get:

Internal Server Error. The server encountered an internal error or misconfiguration and was unable to complete your request.`

I tried deleting wp-config.php and it doesn't change anything. It's like this file isn't being read by my WP installation. I also tried enabling debugging with

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);

but my error_log file only shows old logs from two weeks ago.

What step am I missing? It's like my wp-config.php isn't even being read at all! Nothing I change on it makes any difference.