again: An unexpected error occurred. Something may be wrong with www.remarpro.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.remarpro.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.remarpro.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)