title: Microsoft Graph API Unknown Error when retrieving user's calendars - Client Credentials Flow
body: <p>I'm trying to get a user's calendars list using Microsoft Graph via Postman. I am using Client Credentials flow and I have Admin Consent for these permissions:</p>
  
  <pre><code>Calendars.Read
  Calendars.ReadWrite
  Directory.Read.All
  Directory.ReadWrite.All
  Mail.Read
  Mail.ReadWrite
  User.Read.All
  User.ReadWrite.All
  </code></pre>
  
  <p>When calling:</p>
  
  <pre><code>https://graph.microsoft.com/v1.0/users/{my_user_id_here}/calendars
  </code></pre>
  
  <p>I'm getting this response:</p>
  
  <pre><code>{
      "error": {
          "code": "UnknownError",
          "message": "",
          "innerError": {
              "request-id": "b0214447-98f6-42e7-9424-d3ee4862a25b",
              "date": "2018-09-11T09:05:05"
          }
      }
  }
  </code></pre>
  
  <p>Also, the HTTP status code is 401. I would say that I have all the required permissions. What am I missing?</p>