Hi @Cederic ,
Great to know that it works now and thanks for sharing the update here.
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
[Graph List Calendars only returns 10 calendars]
Issue Symptom:
have a problem to retrieve all calendars from a shared mailbox.
Current status:
The issue has been resolved by removing delegated permission.
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks for your understanding!