Hello @Kloth, Ralf
Looking at the exception details, it seems like there could be a possibility that the actual subscription was originally removed, and notifications are not being sent. Sometimes, subscriptions could be orphaned in the system. For this, we need to wait for the subscription to be removed properly from the system. Also, please note that the "free" azure accounts don't have teams' licenses, one guess that the subscription call will fail with something.
In case if you are still experiencing this issue, I would recommend you to raise a support case with Microsoft Graph, a Support Engineer will be able to assist you better. You can raise support ticket from http://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
Hope this helps.
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