Hello @EdEntw ,
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To create an event in the shared calendar try using the POST users/{email}/calendars/{id}/events
API, please ensure the user has the necessary permissions to edit the shared calendar.
The 500 error is a server-side issue that can arise from various factors, such as internal server errors, incorrect API request formats, or problems with the requested resources. If the error is intermittent and the request succeeds upon retry, it might indicate a temporary server issue or a transient network problem. I have successfully created an event in the shared calendar with the required access. You can refer to the screenshot below.
You could wait for some time and then test again. If the issue isn't fixed, try calling the API in the Graph Explorer or Postman with a user who has permission to edit this shared calendar. If the issue persists, I suggest submitting a ticket in the admin center with the request ID and timestamp so you can get detailed error information.
Documentation reference: https://learn.microsoft.com/en-us/graph/api/calendar-post-events?view=graph-rest-1.0&tabs=http
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