Nextcloud doesn't load CSS after upgrading

Nextcloud version (eg, 18.0.2): 15.0.12
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04.02 LTS
Apache or nginx version (eg, Apache 2.4.25): Nginx 1.16.1
PHP version (eg, 7.1): PHP 7.3

The issue you are facing:

Is this the first time you’ve seen this error? (Y/N): YES

Steps to replicate it:

  1. Access the login screen
  2. Press login button
  3. Access the main panel and does not load CSS

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'instanceid' => 'instante',
  'passwordsalt' => 'password',
  'secret' => 'secretpassword',
  'trusted_domains' =>
  array (
    0 => 'drive.mysite.es',
  ),
  'datadirectory' => '/var/www/drive.mysite.es/data',
  'dbtype' => 'mysql',
  'version' => '15.0.12.1',
  'overwrite.cli.url' => 'https://drive.mysite.es',
  'dbname' => 'mysite',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'uS3rNxtKl0uD',
  'dbpassword' => 'Pkdskajq4334KJNkjnd',
  'installed' => true,
  'mail_smtpmode' => 'smtp',
  'mail_from_address' => 'drive',
  'mail_domain' => 'clickdatos.es',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtphost' => 'servidor.mysite.es',
  'mail_smtpport' => '25',
  'mail_smtpname' => 'drive@mysite.es',
  'mail_smtppassword' => 'dmI1drobG6',
  'memcache.local' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' =>
  array (
    'host' => 'localhost',
    'port' => 6379,
  ),
  'maintenance' => false,
  'loglevel' => 2,
);

The output of your Apache/nginx/system log in /var/log/____:

2020/11/02 16:52:19 [error] 5135#5135: *1 FastCGI sent in stderr: "Primary script unknown" while reading response header from upstream, client: 79.148.133.250, server: drive.mysite, request: "GET /public/phpinfo.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.3-fpm.sock:", host: "drive.mysite.es"

Nginx file:

server {
add_header X-Frame-Options SAMEORIGIN;
server_name drive.mysite.es;
index index.php index.html;
access_log /var/log/nginx/drive.mysite.es/access.log;
error_log /var/log/nginx/drive.mysite.es/error.log;
root /var/www/drive.mysite.es;
include snippets/wordpress7.3.conf;

location ~ .php$ {
include snippets/fastcgi-php.conf;
fastcgi_pass unix:/run/php/php7.3-fpm.sock;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
include fastcgi_params;
}

listen [::]:443 ssl; # managed by Certbot
listen 443 ssl; # managed by Certbot
ssl_certificate /etc/letsencrypt/live/drive.mysite.es/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/drive.mysite.es/privkey.pem; # managed by Certbot
include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}

server {
if ($host = drive.mysite.es) {
return 301 https://$host$request_uri;
} # managed by Certbot

    listen 80;
    listen [::]:80;
    server_name drive.mysite.es;
return 404; # managed by Certbot

}