Don't forget the nginx proxy pass configuration

nginx.conf

# nginx conf        
location / {
        proxy_pass https://example.com/matomo/; # or without the 'matomo' path, but when with path, don't forget the trailing '/'
        proxy_set_header Host             example.com;
        proxy_set_header X-Real-IP        $remote_addr;
        proxy_set_header X-Forwarded-For  $proxy_add_x_forwarded_for;
        proxy_set_header X-Accel-Internal /internal-nginx-static-location;
        access_log off;
}