FPM Container in subdirectory of nginx proxy

I do want to host the nextcloud as a docker container with an nginx as proxy container and the nextcloud as fpm container. If I run the setup as intended under the url https://laburl it works as intended. If I change the url to a sub directory i get multiple 404 errors. I suspect that it is due to the wrong PHP parameters, but I cannot say exactly where the error lies.

Nginx config:

...
http {
...
    upstream php-handler {
        server app:9000;
    }
    map $arg_v $asset_immutable {
       "" "";
    default "immutable";
    }
    server {
        listen 443 ssl;
        http2 on;

        server_tokens off;

        ssl_certificate /etc/nginx/ssl/fullchain.pem;
        ssl_certificate_key /etc/nginx/ssl/privkey.pem;
    
        fastcgi_hide_header X-Powered-By;
        add_header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload";
        add_header Referrer-Policy                      "no-referrer"   always;
        add_header X-Content-Type-Options               "nosniff"       always;
        add_header X-Download-Options                   "noopen"        always;
        add_header X-Frame-Options                      "SAMEORIGIN"    always;
        add_header X-Permitted-Cross-Domain-Policies    "none"          always;
        add_header X-Robots-Tag                         "noindex,nofollow"          always;
        add_header X-XSS-Protection                     "1; mode=block" always;

        root /var/www/html/;
   
        location = /robots.txt {
                allow all;
                log_not_found off;
                access_log off;
        }

        location ^~ /.well-known {
                # The rules in this block are an adaptation of the rules
                # in the Nextcloud `.htaccess` that concern `/.well-known`.

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

                location /.well-known/acme-challenge    { try_files $uri $uri/ =404; }
                location /.well-known/pki-validation    { try_files $uri $uri/ =404; }

                # Let Nextcloud's API for `/.well-known` URIs handle all other
        }
       location ^~ /nextcloud {
        # set max upload size and increase upload timeout:
        client_max_body_size 512M;
        client_body_timeout 300s;
        fastcgi_buffers 64 4K;

        # Enable gzip but do not remove ETag headers
        gzip on;
        gzip_vary on;
        gzip_comp_level 4;
        gzip_min_length 256;
        gzip_proxied expired no-cache no-store private no_last_modified no_etag auth;
        gzip_types application/atom+xml application/javascript application/json application/ld+json application/manifest+json application/rss+xml application/vnd.geo+json application/vnd.ms-fontobject application/wasm application/x-font-ttf application/x-web-app-manifest+json application/xhtml+xml application/xml font/opentype image/bmp image/svg+xml image/x-icon text/cache-manifest text/css text/plain text/vcard text/vnd.rim.location.xloc text/vtt text/x-component text/x-cross-domain-policy;

        # Pagespeed is not supported by Nextcloud, so if your server is built
        # with the `ngx_pagespeed` module, uncomment this line to disable it.
        #pagespeed off;

        # The settings allows you to optimize the HTTP2 bandwitdth.
        # See https://blog.cloudflare.com/delivering-http-2-upload-speed-improvements/
        # for tunning hints
        client_body_buffer_size 512k;

        # HTTP response headers borrowed from Nextcloud `.htaccess`
        add_header Referrer-Policy                   "no-referrer"       always;
        add_header X-Content-Type-Options            "nosniff"           always;
        add_header X-Download-Options                "noopen"            always;
        add_header X-Frame-Options                   "SAMEORIGIN"        always;
        add_header X-Permitted-Cross-Domain-Policies "none"              always;
        add_header X-Robots-Tag                      "noindex, nofollow" always;
        add_header X-XSS-Protection                  "1; mode=block"     always;

        # Remove X-Powered-By, which is an information leak
        fastcgi_hide_header X-Powered-By;

        # Specify how to handle directories -- specifying `/nextcloud/index.php$request_uri`
        # here as the fallback means that Nginx always exhibits the desired behaviour
        # when a client requests a path that corresponds to a directory that exists
        # on the server. In particular, if that directory contains an index.php file,
        # that file is correctly served; if it doesn't, then the request is passed to
        # the front-end controller. This consistent behaviour means that we don't need
        # to specify custom rules for certain paths (e.g. images and other assets,
        # `/updater`, `/ocm-provider`, `/ocs-provider`), and thus
        # `try_files $uri $uri/ /nextcloud/index.php$request_uri`
        # always provides the desired behaviour.
        index index.php index.html /nextcloud/index.php$request_uri;

        # Rule borrowed from `.htaccess` to handle Microsoft DAV clients
        location = /nextcloud {
            if ( $http_user_agent ~ ^DavClnt ) {
                return 302 /nextcloud/remote.php/webdav/$is_args$args;
            }
        }

        # Rules borrowed from `.htaccess` to hide certain paths from clients
        location ~ ^/(?:build|tests|config|lib|3rdparty|templates|data)(?:$|/)    { return 404; 
        }
        location ~ ^/(?:\.|autotest|occ|issue|indie|db_|console)                  { return 404; 
        }

        # Ensure this block, which passes PHP files to the PHP process, is above the blocks
        # which handle static assets (as seen below). If this block is not declared first,
        # then Nginx will encounter an infinite rewriting loop when it prepends
        # `/nextcloud/index.php` to the URI, resulting in a HTTP 500 error response.
        location ~ \.php(?:$|/) {
            # Required for legacy support
            rewrite ^/nextcloud/(?!index|remote|public|cron|core\/ajax\/update|status|ocs\/v[12]|updater\/.+|oc[ms]-provider\/.+|.+\/richdocumentscode\/proxy) index.php$request_uri;

            fastcgi_split_path_info ^(.+?\.php)(/.*)$;
            set $path_info $fastcgi_path_info;

            #try_files $fastcgi_script_name =404;

            include fastcgi_params;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            #fastcgi_param SCRIPT_FILENAME /var/www/html/$fastcgi_script_name;
            fastcgi_param PATH_INFO $path_info;
            fastcgi_param HTTPS on;

            fastcgi_param modHeadersAvailable true;         # Avoid sending the security headers twice
            fastcgi_param front_controller_active true;     # Enable pretty urls
            fastcgi_pass php-handler;

            fastcgi_intercept_errors on;
            fastcgi_request_buffering off;

            fastcgi_max_temp_file_size 0;
            alias /var/www/html/;
        }

        location ~ \.(?:css|js|svg|gif|png|jpg|ico|wasm|tflite|map)$ {
            try_files $uri /nextcloud/index.php$request_uri;
            add_header Cache-Control "public, max-age=15778463, $asset_immutable";
            access_log off;     # Optional: Don't log access to assets

            location ~ \.wasm$ {
                default_type application/wasm;
            }
        }

        location ~ \.woff2?$ {
            try_files $uri /nextcloud/index.php$request_uri;
            expires 7d;         # Cache-Control policy borrowed from `.htaccess`
            access_log off;     # Optional: Don't log access to assets
        }

        # Rule borrowed from `.htaccess`
        location /nextcloud/remote {
            return 301 /nextcloud/remote.php$request_uri;
        }

        location /nextcloud {
            try_files $uri $uri/ /nextcloud/index.php$request_uri;
        }
    } 
}
}

Logs of nextcloud FPM container:

172.19.0.6 -  21/Mar/2024:14:03:42 +0000 "GET /nextcloud/index.php" 404

Logs of nginx container:

10.128.6.135 - - [21/Mar/2024:14:03:42 +0000] "GET /nextcloud/index.php HTTP/2.0" 404 36 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:123.0) Gecko/20100101 Firefox/123.0" "-"
2024/03/21 14:03:43 [error] 30#30: *1 open() "/var/www/html/index.php/apps/files/preview-service-worker.js" failed (20: Not a directory), client: 10.128.6.135, server: , request: "GET /index.php/apps/files/preview-service-worker.js HTTP/2.0", host: "https://url"
10.128.6.135 - - [21/Mar/2024:14:03:43 +0000] "GET /index.php/apps/files/preview-service-worker.js HTTP/2.0" 404 146 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:123.0) Gecko/20100101 Firefox/123.0" "-"