Nginx produces 412 errors

Hi,

since my migration to a new server I´m getting 412 errors at my web browser console from time to time at loading the file list or when I am writing a note:

files.js?v=8a34a280-10:51 POST https://cloud.xy.de/apps/text/session/sync 412
(anonymous) @ files.js?v=8a34a280-10:51
e.exports @ files.js?v=8a34a280-10:51
e.exports @ files.js?v=8a34a280-10:51
Promise.then (async)
c.request @ files.js?v=8a34a280-10:51
r.forEach.c.<computed> @ files.js?v=8a34a280-10:51
(anonymous) @ files.js?v=8a34a280-10:51
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.catch (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
Promise.then (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
setInterval (async)
value @ editor.js?v=5145bf0dbc6192145c28:23
(anonymous) @ editor.js?v=5145bf0dbc6192145c28:23
editor.js?v=5145bf0dbc6192145c28:23 Failed to fetch steps due to other reason Error: Request failed with status code 412
    at e.exports (files.js?v=8a34a280-10:51)
    at e.exports (files.js?v=8a34a280-10:51)
    at XMLHttpRequest.d.onreadystatechange (files.js?v=8a34a280-10:51)

I don´t know what I could have missed. My nginx config is:

server {
    listen 443 ssl http2 default_server;
    server_name cloud.xy.de;
    ssl_certificate /etc/letsencrypt/live/cloud.xy.de/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/cloud.xy.de/privkey.pem;
    add_header Strict-Transport-Security "max-age=63072000; preload";
    add_header X-Content-Type-Options nosniff;
    add_header X-Frame-Options "SAMEORIGIN" always;
    add_header X-XSS-Protection "1; mode=block";
    add_header X-Robots-Tag none;
    add_header X-Download-Options noopen;
    add_header X-Permitted-Cross-Domain-Policies none;
    add_header Referrer-Policy "strict-origin";
    root /var/kunden/cloud/httpdocs/nextcloud;

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

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

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

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

    # set max upload size
    client_max_body_size 16400M;
    client_body_buffer_size 2048576k;
    client_body_temp_path /var/ngx_body;
    fastcgi_buffers 64 4K;
    gzip off;

    rewrite ^/caldav(.*)$ /remote.php/caldav$1 redirect;
    rewrite ^/carddav(.*)$ /remote.php/carddav$1 redirect;
    rewrite ^/webdav(.*)$ /remote.php/webdav$1 redirect;

    #pagespeed off;

    location / {
            rewrite ^ /index.php$request_uri;
            rewrite ^/.well-known/host-meta /public.php?service=host-meta last;
            rewrite ^/.well-known/host-meta.json /public.php?service=host-meta-json last;

            rewrite ^/.well-known/carddav /remote.php/carddav/ redirect;
            rewrite ^/.well-known/caldav /remote.php/caldav/ redirect;

            rewrite ^(/core/doc/[^\/]+/)$ $1/index.html;
    }

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

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

    location ~ ^/(?:index|remote|public|cron|core/ajax/update|status|ocs/v[12]|updater/.+|ocs-provider/.+|core/templates/40[34])\.php(?:$|/) {
            fastcgi_split_path_info ^(.+\.php)(/.*)$;
            include fastcgi_params;
            try_files $fastcgi_script_name =404;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            fastcgi_param PATH_INFO $fastcgi_path_info;
            fastcgi_param HTTPS on;
            fastcgi_param modHeadersAvailable true;
            fastcgi_param front_controller_active true;
            fastcgi_pass unix:/var/run/php/php7.3-fpm-cloud.sock;
            fastcgi_intercept_errors on;
            #fastcgi_request_buffering off;
            fastcgi_read_timeout 76000;
            fastcgi_connect_timeout 60;
            fastcgi_send_timeout 180;
            #fastcgi_cache OWNCLOUD;
            #fastcgi_cache_valid  60m;
    }

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

    location ~* \.(?:css|js)$ {
            try_files $uri /index.php$uri$is_args$args;
            add_header Cache-Control "public, max-age=7200";
            add_header X-Content-Type-Options nosniff;
            add_header X-Frame-Options "SAMEORIGIN";
            add_header X-XSS-Protection "1; mode=block";
            add_header X-Robots-Tag none;
            add_header X-Download-Options noopen;
            add_header X-Permitted-Cross-Domain-Policies none;
            access_log off;
    }

    location ~* \.(?:svg|gif|png|html|ttf|woff|ico|jpg|jpeg)$ {
            try_files $uri /index.php$uri$is_args$args;
            access_log off;
}

}

Has anyone an useful hint for me?