Turns out I was simply using a wrong command line to run the playbook: ansible-playbook -i inventory.yml tasks/main.yml instead of ansible-playbook -i inventory.yml playbook.yml.

After that the error messages became much clearer and my final playbook looks like:

---
- name: Play
  hosts: all
  become: true
  roles:
    - /home/marco/play/roles/ics-ans-orca-driver