After upgrading from 11.0.5 to 12.0.3 security error message in log

My installation is on Ubuntu 16.04 using nginx (also as a reverse proxy) and I manually upgraded from 11.0.5 to 12.0.3. Everything works, except this one error message:

Der „X-Frame-Options“-HTTP-Header ist nicht so konfiguriert, dass er „SAMEORIGIN“ entspricht. Dies ist ein potentielles Sicherheitsrisiko und es wird empfohlen, diese Einstellung zu ändern.

This is the VServer-conf from nginx (which contains this SAMEORIGIN setting):

upstream php-handler {
server unix:/run/php/php7.0-fpm.sock;
}

server {
listen 127.0.0.1:82;
server_name 127.0.0.1;

# Add headers to serve security related headers
# Use 'proxy_set_header' (not 'add_header') as the headers have to be passed through a proxy.
proxy_set_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;";
proxy_set_header X-Content-Type-Options nosniff;
proxy_set_header X-Frame-Options "SAMEORIGIN";
proxy_set_header X-XSS-Protection "1; mode=block";
proxy_set_header X-Robots-Tag none;
proxy_set_header X-Download-Options noopen;
proxy_set_header X-Permitted-Cross-Domain-Policies none;

# Path to the root of your installation
root /var/www/;

location = /robots.txt {
    allow all;
    log_not_found off;
    access_log off;
}

# The following 2 rules are only needed for the user_webfinger app.
# Uncomment it if you're planning to use this app.
#rewrite ^/.well-known/host-meta /owncloud/public.php?service=host-meta last;
#rewrite ^/.well-known/host-meta.json /owncloud/public.php?service=host-meta-json last;

location = /.well-known/carddav { return 301 $scheme://$host/nextcloud/remote.php/dav; }
location = /.well-known/caldav { return 301 $scheme://$host/nextcloud/remote.php/dav; }

location /.well-known/acme-challenge { }

location ^~ /nextcloud {

    # set max upload size
    client_max_body_size 1G;
    fastcgi_buffers 64 4K;

    # Disable gzip to avoid the removal of the ETag header
    gzip off;

    # Uncomment if your server is build with the ngx_pagespeed module
    # This module is currently not supported.
    #pagespeed off;

    error_page 403 /nextcloud/core/templates/403.php;
    error_page 404 /nextcloud/core/templates/404.php;

    location /nextcloud {
        rewrite ^ /nextcloud/index.php$uri;
    }

    location ~ ^/nextcloud/(?:build|tests|config|lib|3rdparty|templates|data)/ {
        deny all;
    }

    location ~ ^/nextcloud/(?:\.|autotest|occ|issue|indie|db_|console) {
        deny all;
    }

    location ~ ^/nextcloud/(?:index|remote|public|cron|core/ajax/update|status|ocs/v[12]|updater/.+|ocs-provider/.+|core/templates/40[34])\.php(?:$|/) {
        include fastcgi_params;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;

  # Important: disable HTTPS, otherwise no log in will be possible!
        #fastcgi_param HTTPS on;

        fastcgi_param modHeadersAvailable true; #Avoid sending the security headers twice
        fastcgi_param front_controller_active true;
        fastcgi_pass php-handler;
        fastcgi_intercept_errors on;

        # Raise timeout values.
  # This is especially important when the ownCloud setup runs into timeouts (504 gateway errors)
  fastcgi_read_timeout 300;
  fastcgi_send_timeout 300;
  fastcgi_connect_timeout 300;
  
        # Pass PHP variables directly to PHP.
        # This is usually done in the php.ini. For more flexibility, these variables are configured in the nginx config.
  # All the PHP parameters have to be set in one fastcgi_param. When using more 'fastcgi_param PHP_VALUE' directives, the last one will override all the others.
        fastcgi_param PHP_VALUE "open_basedir=/var/www:/tmp/:/var/clouddata:/dev/urandom
  				upload_max_filesize = 1G
  				post_max_size = 1G
  				max_execution_time = 3600";
        
        # Make sure that the real IP of the remote host is passed to PHP.
        fastcgi_param REMOTE_ADDR $http_x_real_ip;
    }

    location ~ ^/nextcloud/(?:updater|ocs-provider)(?:$|/) {
        try_files $uri/ =404;
        index index.php;
    }

    # Adding the cache control header for js and css files
    # Make sure it is BELOW the PHP block
    location ~* \.(?:css|js)$ {
        try_files $uri /nextcloud/index.php$uri$is_args$args;
        proxy_set_header Cache-Control "public, max-age=7200";
        # Add headers to serve security related headers
        # Again use 'proxy_set_header' (not 'add_header') as the headers have to be passed through a proxy.
        proxy_set_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;";
        proxy_set_header X-Content-Type-Options nosniff;
        proxy_set_header X-Frame-Options "SAMEORIGIN";
        proxy_set_header X-XSS-Protection "1; mode=block";
        proxy_set_header X-Robots-Tag none;
        proxy_set_header X-Download-Options noopen;
        proxy_set_header X-Permitted-Cross-Domain-Policies none;
        # Optional: Don't log access to assets
        access_log off;
    }

    location ~* \.(?:svg|gif|png|html|ttf|woff|ico|jpg|jpeg)$ {
        try_files $uri /nextcloud/index.php$uri$is_args$args;
        # Optional: Don't log access to other assets
        access_log off;
    }
}

}

Here is my config.php:

?php
$CONFIG = array (
‘instanceid’ => ‘hereisinstanceid’,
‘passwordsalt’ => ‘hereisapassword/l’,
‘secret’ => ‘hereissecret’,
‘trusted_domains’ =>
array (
0 => ‘mydomain.com’,
1 => ‘192.168.40.132’,
),
‘datadirectory’ => ‘/var/clouddata’,
‘overwrite.cli.url’ => ‘https://mydomain.com/nextcloud’,
‘dbtype’ => ‘mysql’,
‘version’ => ‘12.0.3.3’,
‘dbname’ => ‘nextcloud_db’,
‘dbhost’ => ‘localhost’,
‘dbport’ => ‘’,
‘dbtableprefix’ => ‘oc_’,
‘dbuser’ => ‘nextcloud_db_user’,
‘dbpassword’ => ‘myDBpassword’,
‘logtimezone’ => ‘Europe/Berlin’,
‘installed’ => true,
‘memcache.local’ => ‘\OC\Memcache\APCu’,
‘maintenance’ => false,
‘mail_smtpmode’ => ‘smtp’,
‘mail_smtpsecure’ => ‘ssl’,
‘mail_from_address’ => ‘noreply’,
‘mail_domain’ => ‘mydomain.com’,
‘mail_smtpauth’ => 1,
‘mail_smtpauthtype’ => ‘LOGIN’,
‘mail_smtpname’ => ‘noreply@mydomain.com’,
‘mail_smtppassword’ => ‘smtppasswort’,
‘mail_smtphost’ => ‘smtp.mailserver.com’,
‘mail_smtpport’ => ‘465’,
‘loglevel’ => 2,
‘updater.release.channel’ => ‘production’,
);

Maybe the reverse-proxy is the culprit? What do I have to change?