Failed to update Nextcloud via docker-compose

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Nextcloud version : 18.0.2.2
Operating system and version _: Ubuntu 18.04.4
Apache or nginx version : fpm-alpine latest
PHP version (eg, 7.1): Donot know

The issue you are facing:

Is this the first time you’ve seen this error? (Y/N): Y

Hello everyone,

I am using docker-compose for my nextcloud setup. I forgot to update my nextcloud for a while and wanted to do it. I did use these commands:

docker-compose pull
docker-compose up -d

My yml file looks like this:

version: '3'

services:
  db:
    image: mariadb
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    restart: always
    volumes:
      - db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=hiddenpw
    env_file:
      - db.env
    container_name: new_nextcloud_db

  app:
    image: nextcloud:fpm-alpine
    restart: always
    volumes:
      - nextcloud:/var/www/html
    environment:
      - MYSQL_HOST=db
    env_file:
      - db.env
    depends_on:
      - db
    container_name: new_nextcloud_app

  web:
    build: ./web
    restart: always
    volumes:
      - nextcloud:/var/www/html:ro
    environment:
      - VIRTUAL_HOST=hiddendomain
      - LETSENCRYPT_HOST=hiddenhost
      - LETSENCRYPT_EMAIL=hiddenemail
    depends_on:
      - app
    networks:
      - proxy-tier
      - default
    container_name: new_nextcloud_web

Since then, my clients cannot connect and I get also this message:

Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

Sadly I fail to know where the server log is saved. But I get also another message via ssh:

Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
Set log level to debug
Exception: Updates between multiple major versions and downgrades are unsupported.
Update failed
Maintenance mode is kept active

I am sure that before my update I didn’t use nextcloud 17 or 16. I think it was version 12.

So I tried downgrading but I failed.

My questions are:
Can I downgrade and progressively upgrade until I reach version 18?
Is there another solution?
Since I know that error logs are key for problem solving, can anybody give me an advice where to find my errorlogs when using docker compose? Than I could publish them here.

Thank you