Why does phpMyAdmin show an almost empty page instead of the login page?

I installed phpMyAdmin with Nginx and MariaDB on my FreeBSD server at home. When I visit the login page, I see this instead of a login page:

Here are the versions of the different components:

• FreeBSD 15.0
• Nginx 1.30.4
• MariaDB 12.3.2
• PHP 8.6.0alpha1

usr/local/etc/mysql/my.cnf contains the following (plus commented out lines):

[client-server]
port    = 3306
socket  = /var/run/mysql/mysql.sock

I added the following in /usr/local/www/phpMyAdmin/config.inc.php:

$cfg['Servers'][$i]['socket'] = '/var/run/mysql/mysql.sock';

The config block I added for Nginx is the following:

server {
        listen 80;
        #server_name app.example.com;
        root /usr/local/www/phpMyAdmin;

        index index.php index.html index.htm;

        location / {
                try_files $uri $uri/ =404;
        }

        location ~ \.php$ {
                include fastcgi_params;
                fastcgi_pass 127.0.0.1:9000;
                fastcgi_index index.php;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }

        error_page 404 /404.html;
        error_page 500 502 503 504 /50x.html;
        location = /50x.html {
                root /usr/local/www/phpMyAdmin;
        }
}

The logs don't contain anything useful to figure out what's wrong. I tried with different versions of php and phpMyAdmin and different browsers, but I get the same result. I searched online and didn't find any post describing the same problem.

What am I missing?