Can't get Nextcloud, Collabora to work with Mattermost

Hi everyone,
Im trying to set up a Nextcloud server with Collabora and Mattermost. Im a Computer Science teacher and want to set this up for my school. I got a VPS running Ubuntu 20.04 for this with the domains office.xxx.eu, teams.xxx.eu and nextcloud.xxx.eu and ip4 11.11.11.11

First I had problems installing mattermost on my own. So I used the
Mattermost Omnibus installation method it was working fine. But if im trying to set up Nextcloud afterwards, its not running and I get a 502 error all the time. I also tried it the other way arround. So I installed Nextcloud following this tutorial and i managed to install collabora with this tutorial. Both worked, but after I installed Mattermost, nothing was working anymore. Same 502 error.

So i could really need someones help.
Since mattermost omnibus is using nginx, PostgreSQL and certbot and i used this for nextcloud aswell with this tutorial.

The Nginx config for mattermost looks like this:

upstream backend {
   server 127.0.0.1:8065;
   keepalive 32;
}

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=mattermost_cache:10m max_size=3g inactive=120m use_temp_path=off;

server {
  listen 80 default_server;
  listen [::]:80 default_server;
  server_name teams.xxx.eu;
  return 301 https://$server_name$request_uri;
}

server {
        listen 443 ssl http2;
      listen [::]:443 ssl http2;
  
    server_name teams.cgs-da.eu;
  
    ssl_certificate /etc/letsencrypt/live/teams.xxx.eu/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/teams.xxx.eu/privkey.pem;
  ssl_session_timeout 1d;
  ssl_protocols TLSv1.2;
  ssl_ciphers 'ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256';
  ssl_prefer_server_ciphers on;
  ssl_session_cache shared:SSL:50m;
    # HSTS (ngx_http_headers_module is required) (15768000 seconds = 6 months)
  add_header Strict-Transport-Security max-age=15768000;

  location ~ /api/v[0-9]+/(users/)?websocket$ {
      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection "upgrade";
      client_max_body_size 50M;
      proxy_set_header Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header X-Forwarded-Proto $scheme;
      proxy_set_header X-Frame-Options SAMEORIGIN;
      proxy_buffers 256 16k;
      proxy_buffer_size 16k;
      client_body_timeout 60;
      send_timeout 300;
      lingering_timeout 5;
      proxy_connect_timeout 90;
      proxy_send_timeout 300;
      proxy_read_timeout 90s;
      proxy_pass http://backend;
  }

  location ~ /plugins/focalboard/ws/* {
      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection "upgrade";
      client_max_body_size 50M;
      proxy_set_header Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header X-Forwarded-Proto $scheme;
      proxy_set_header X-Frame-Options SAMEORIGIN;
      proxy_buffers 256 16k;
      proxy_buffer_size 16k;
      client_body_timeout 60;
      send_timeout 300;
      lingering_timeout 5;
      proxy_connect_timeout 90;
      proxy_send_timeout 300;
      proxy_read_timeout 90s;
      proxy_pass http://backend;
  }

  location / {
      client_max_body_size 50M;
      proxy_set_header Connection "";
      proxy_set_header Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header X-Forwarded-Proto $scheme;
      proxy_set_header X-Frame-Options SAMEORIGIN;
      proxy_buffers 256 16k;
      proxy_buffer_size 16k;
      proxy_read_timeout 600s;
      proxy_cache mattermost_cache;
      proxy_cache_revalidate on;
      proxy_cache_min_uses 2;
      proxy_cache_use_stale timeout;
      proxy_cache_lock on;
      proxy_http_version 1.1;
      proxy_pass http://backend;
  }
}


When I install Nextcloud afterwards the nginx looks like this:

server {
    listen 80;
    listen [::]:80;
    server_name nextcloud.xxx.com;

    # Add headers to serve security related headers
    add_header X-Content-Type-Options nosniff;
    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 no-referrer;

    #I found this header is needed on Ubuntu, but not on Arch Linux. 
    add_header X-Frame-Options "SAMEORIGIN";

    # Path to the root of your installation
    root /usr/share/nginx/nextcloud/;

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

    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 /public.php?service=host-meta last;
    #rewrite ^/.well-known/host-meta.json /public.php?service=host-meta-json
    # last;

    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 {
      allow all;
    }

    # set max upload size
    client_max_body_size 512M;
    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 /core/templates/403.php;
    error_page 404 /core/templates/404.php;

    location / {
       rewrite ^ /index.php;
    }

    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(?:$|/) {
       include fastcgi_params;
       fastcgi_split_path_info ^(.+\.php)(/.*)$;
       try_files $fastcgi_script_name =404;
       fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
       fastcgi_param PATH_INFO $fastcgi_path_info;
       #Avoid sending the security headers twice
       fastcgi_param modHeadersAvailable true;
       fastcgi_param front_controller_active true;
       fastcgi_pass unix:/run/php/php7.4-fpm.sock;
       fastcgi_intercept_errors on;
       fastcgi_request_buffering off;
    }

    location ~ ^/(?: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 $uri /index.php$uri$is_args$args;
        add_header Cache-Control "public, max-age=7200";
        # Add headers to serve security related headers (It is intended to
        # have those duplicated to the ones above)
        add_header X-Content-Type-Options nosniff;
        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 no-referrer;
        # Optional: Don't log access to assets
        access_log off;
   }

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

and with this i get a 502 Bad Gateway error. Maybe it has to do with the ports or i did something else wrong.

Let me know if I can provide more information.
Thanks for your help in advance.

Cheers,
geE68