Can't get into outlook

The ADTransientException during StartupData is an Outlook on the web server response, so the Microsoft 365 administrator should first check the tenant’s Exchange Online service health and then run the Outlook connectivity diagnostic if no incident is listed.

1. Check Exchange Online service health

Ask your Microsoft 365 administrator to:

1. Sign in to the Microsoft 365 admin center.
2. Open Health → Service health.
3. Review Exchange Online under Active issues Microsoft is working on.
4. Also check Issues for your organization to act on.
5. Open any matching incident or advisory and follow its updates or workaround.

The tenant-specific dashboard is more detailed than the public status page. If the problem is not listed, the administrator can select Report an issue from Service health. ^1^

If the admin center itself cannot be opened, check the public Microsoft service-status page instead. ^2^

2. If no incident is shown

Have the administrator run Microsoft’s service-side connectivity checks:

1. In the Microsoft 365 admin center, select Help & support.
2. Enter Outlook user connectivity.
3. Run the diagnostic for the affected account.
4. Apply any reported remediation.

This diagnostic checks possible service-side settings affecting Outlook connectivity. Admin-center diagnostics do not make tenant changes without consent. They are unavailable in GCC High, DoD, and Microsoft 365 operated by 21Vianet. ^3^

As a quick browser-side discriminator, try one InPrivate or Incognito window. If Outlook works there, clear Outlook/Microsoft site cookies and cache for all time. If the same StartupData error appears there, proceed with admin escalation rather than repeatedly clearing browser data. ^4^

3. Escalate the mailbox failure

If the diagnostic does not fix it and no service incident is posted, the administrator should:

1. Open the Microsoft 365 admin center.
2. Select Help & support in the lower-right corner.
3. Describe the issue as:
   Outlook on the web fails during StartupData with HTTP 500 and Microsoft.Exchange.Data.Directory.ADTransientException
4. If the suggested results do not resolve it, select Contact Support and choose phone or email. ^5^
5. Privately provide Support with:
   • UTC timestamp
   • Request ID
   • Session ID
   • Client version
   • Front-end server value
   • Correlation value
   • Full exception name

Do not repost those identifiers publicly. If the subscription was purchased entirely through a Microsoft partner, contact that partner to open the support case. If you are not an administrator, send the error details to your organization’s Microsoft 365 administrator.

The Exchange Server update-reinstallation fix does not apply unless your organization runs on-premises Exchange and this began immediately after installing an Exchange security update; it is not a repair procedure for Exchange Online. ^6^

References

1. How to check Microsoft 365 service health
2. Microsoft service health status
3. Self-help diagnostics for issues in Exchange Online and Outlook
4. how to fix outlook website something went wrong - Microsoft Q&A
5. Get support for Microsoft 365 for business
6. Fix failed Exchange Server updates