terraform registry from gitlab - authentication issues

Hello,

i am trying to setup dependabot-gitlab and have merge requests created when i update a terraform module coming from another project.

For that i created the dependabot.yaml and can run the job just fine. It doesn't fail, even though it is unable to reach my gitlabs infrastructure registry.

The registry is defined as:

version: 2
registries:
  terraform-gitlab:
    type: terraform-registry
    url: https://my-gitlab.my-domain.tld
    token: ${{CI_REGISTRY_PASSWORD}}
updates:
  - package-ecosystem: terraform
    directory: "/build/infrastructure/env/sandbox"
    schedule:
      interval: "daily"
    open-pull-requests-limit: 20
    rebase-strategy: auto
    commit-message:
      prefix: "refs #0: terraform module update;\n    - "
    registries:
      - terraform-gitlab

As we are using a very recent version of gitlab, the $CI_REGISTRY_PASSWORD is set (confirmed via separate ci job).

The job however outputs:

[2021-12-17 13:24:07 +0000 tid=1afe] INFO: [my-group/my-project=>terraform=>/build/infrastructure/env/sandbox] Fetching info for my-group/tf-module/test/aws
[2021-12-17 13:24:07 +0000 tid=1afe] ERROR: [my-group/my-project=>terraform=>/build/infrastructure/env/sandbox] The following source could not be reached as it requires authentication (and any provided details were invalid or lacked the required permissions): my-gitlab.my-domain.tld

I did verify that i as the user who created the scheduled pipeline can access the terraform registry of the used gitlab instance.

Am i missing something?