Hello Harald Gächter,
Thanks for posting!
As per my testing, I am also able to replicate the behavior for the meeting when I'm not an organizer or in the participation list. As it's expected behavior for the API, so it'll not return any data in the API response.
Note: - To use application permission for this API, tenant administrators must create an application access policy and grant it to a user to authorize the app configured in the policy to fetch online meetings and/or online meeting artifacts on behalf of that user (with user ID specified in the request path).
Hope this helps.
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