user is unable to open the calender app inside the teams

Check Microsoft 365 Service health and then open a Microsoft support request for a user-scoped Teams–Exchange Online integration failure, because the Calendar request is returning HTTP 429 across Teams desktop, Teams web, new Outlook, browsers, and networks while the documented prerequisites are already satisfied.

1. Check for an active service incident

In the Microsoft 365 admin center, go to:

Health → Service health

Review both Microsoft Teams and Exchange Online, including:

• Active issues Microsoft is working on
• Issues for your organization to act on
• Issue history for the time the problem began

Microsoft recommends checking this page before further troubleshooting cloud-service failures.^1^

If an incident matches Teams Calendar or Teams meeting creation in Outlook, monitor that incident rather than changing the mailbox configuration.

2. Escalate with the HTTP 429 evidence

If Service health shows no matching incident, open a support request from the Microsoft 365 admin center. Include:

1. The affected user’s UPN.
2. The exact reproduction time in UTC.
3. The error text copied through Copy error information on the Teams Calendar error page.
4. The Teams web HAR/network trace containing the failed Calendar request.
5. For the HTTP 429 response:
   • Full request URL, with sensitive query values removed.
   • Retry-After header, if present.
   • request-id, client-request-id, x-ms-*, correlation, and diagnostic headers.
   • Response body.
6. Teams desktop debug logs captured immediately after reproducing the issue.
7. The complete Remote Connectivity Analyzer report showing mailbox discoverability/EWS or REST failure.
8. The start time and last-known-good time.
9. Confirmation that:
   • Only one user or a defined subset is affected.
   • Outlook Calendar itself opens.
   • Classic Outlook can create the Teams meeting.
   • Teams desktop/web and new Outlook/OWA fail.
   • Different browsers and networks produce the same result.
   • Autodiscover returns the Exchange Online EWS endpoint.
   • EWS is enabled with no allow/block policy.

Microsoft’s documented escalation path specifically requests the affected UPN, UTC reproduction time, and Teams debug logs after prerequisites and configuration have been validated.^2^

HTTP 429 means the API request was throttled or rate-limited; a Retry-After header can specify when the client should retry.^3^ In this case, the response and correlation headers are important because an administrator cannot directly adjust the request behavior of Microsoft’s Teams Calendar client.

Why further EWS changes are not indicated

The Teams Calendar requires access to the Exchange mailbox through EWS.^2^ However, the relevant checks have already passed:

• Mailbox and organization EwsEnabled are True.
• No EWS application access policy, allow list, or block list is configured.
• Autodiscover returns the correct Exchange Online EWS endpoint.
• The Calendar app is assigned.
• Coexistence mode is Teams Only.
• The mailbox and license are active.

Therefore, do not add an EWS allow list, reassign the license, reinstall Teams, clear client caches, or repeat browser/network testing unless Microsoft Support identifies a specific reason. Those actions do not address the captured server response and could introduce unrelated configuration changes.

As a temporary workaround, continue scheduling through classic Outlook, where Teams meeting creation is currently functioning. New Outlook and Outlook on the web use native Teams meeting integration rather than the classic Outlook add-in, which accounts for the different behavior between those clients.^4^

References

1. How to check Microsoft 365 service health
2. Resolve interaction issues between Teams and Exchange Server
3. What is throttling?
4. Schedule a Microsoft Teams meeting from Outlook