again: An unexpected error occurred. Something may be wrong with www.ads-software.com o

I read countless articles about this error message and potential ways to solve this. Nothing helped me. I could need help to figure to what the problem might be.

My situation:

I have a nginx installation on my debian server. I configured a general nginx.conf (http settings) and I have a list of sites-enabled. My nginx does basically the https termination for a couple of applications on my server. Now I installed wordpress in a docker environment.

I used the following docker compose file.

version: "3.9"

services:
  db:
    image: mysql:8.0
    container_name: db_wp
    restart: always
    environment:
      - MYSQL_DATABASE=wordpress
      - MYSQL_ROOT_PASSWORD=<MYPASSWORD>
      - MYSQL_USER=wordpress
      - MYSQL_PASSWORD=<ANOTHER_PASSWORD>
    volumes:
      - /opt/docker/instances/wordpress/dbdata:/var/lib/mysql

  wordpress:
    depends_on:
      - db
    image: wordpress:latest
    container_name: wordpress
    ports:
      - 127.0.0.1:9000:80
    restart: always
    environment:
      - WORDPRESS_DB_HOST=db:3306
      - WORDPRESS_DB_USER=wordpress
      - WORDPRESS_DB_PASSWORD=wordpress
      - WORDPRESS_DB_NAME=wordpress
    volumes:
      - /opt/docker/instances/wordpress/data:/var/www/html

volumes:
  db_data: {}

I configured Nginx to forward https to http//localhost:9000.

After completed startup I went to the server name where wordpress should be reachable, the installation screen came up asking for my language and I let the installation complete.

After that I had to login and was brought to the wp-admin page. Everything exactly as it should be.

Then I went to the themes config section, clicked on “Add new Theme” and I got his message:

An unexpected error occurred. Something may be wrong with www.ads-software.com or this server’s configuration. If you continue to have problems, please try the support forums.

The same as many posts complained about.

What did I checkout so far?

I went to the plugin config section and clicked on Add new Plugin. It worked. No Problems at all

By default, 3 Themes were provided after the installation. Instead of trying to install a new one I clicked on update on one of them. After a short while it said: Update succesfull

I went into the docker container (docker exec …..) of wordpress and did a network test with curl to api.www.ads-software.com. It is reachable

Everything seems to work perfectly but I still get this error for adding a new theme.

I also added a WP_DEBUG to my config but no debug is produced:

define( ‘WP_DEBUG’, true );

define( ‘WP_DEBUG_LOG’, ‘/var/www/html/logs/problems.log’) ;

No log is written.

Any ideas what could be my problem ? (I doubt that my problem is nginx related so I did not provide its config here)