title: Error on adding subscription on events using MS Graph
body: <p>I am using the MS graph subscription endpoint (<a href="https://graph.microsoft.com/v1.0/subscriptions" rel="nofollow noreferrer">https://graph.microsoft.com/v1.0/subscriptions</a>) to add a subscription on a calendars events for some of my end users outlook accounts. For some accounts (not all) this fails with this error:</p>
  <pre><code>{
    &quot;error&quot;: {
      &quot;code&quot;: &quot;ExtensionError&quot;,
      &quot;message&quot;: &quot;Operation: Create; Exception: [Status Code: ServiceUnavailable; Reason: Target resource 'xxxx-xxx-xxx-xxxx' hosted on database 'xxxx-xxx-xxx-xxxx' is currently on backend 'Unknown']&quot;,
      &quot;innerError&quot;: {
        &quot;date&quot;: &quot;2021-05-31T08:27:37&quot;,
        &quot;request-id&quot;: &quot;c7c38d7a-617c-4601-9684-0f924e86248a&quot;,
        &quot;client-request-id&quot;: &quot;c7c38d7a-617c-4601-9684-0f924e86248a&quot;
      }
    }
  }
  </code></pre>
  <p>Just before adding the subscription my code have been able to add a new event to the same calendar, so I think that my access token has the right permissions to access the calendar. So I don't understand how I can get an &quot;ExtensionError&quot;?</p>
  <p>Here is the C# code that I run to try to add the subscription:</p>
  <pre><code>var subscription = new Subscription
  {
      ChangeType = &quot;created,updated,deleted&quot;,
      NotificationUrl = GetNotificationUrl(),
      Resource = $&quot;me/calendars/{calendarId}/events&quot;,
      ExpirationDateTime = DateTimeOffset.Now.AddHours(70),
      ClientState = clientState,
  };
  
  await graphServiceClient
      .Subscriptions
      .Request()
      .Header(&quot;Prefer&quot;, $&quot;IdType=\&quot;ImmutableId\&quot;&quot;)
      .AddAsync(subscription);
  </code></pre>