I tested this endpoint and its working as expected.
I would suggest you to get Event-ID using GET /me/events Graph API endpoint. Please refer to the below sample screenshot.
Using this Graph API endpoint you will be able to get list of event objects in the user's mailbox. Please refer to this documentation for more details.
After passing Event-ID in GET /me/events/{id} endpoint, we can get the details of a specific event in API response.
Reference Screenshot:
However if still this issue persists, 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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