HTTP ERROR 500

Open Outlook on the web in an InPrivate or Incognito window first; if it works there, clear the browser’s Outlook/Microsoft 365 cookies and cached data, then sign in again. This isolates a browser-session problem without changing the mailbox. ^1^

If the HTTP 500 continues

1. Try another supported browser or device.
   • If Outlook opens, the problem is local to the original browser.
   • Clear cookies and cached files for Microsoft 365/Outlook in that browser, close it completely, and reopen it.
2. Determine whether other employees are affected.
   • Several users affected: ask a Microsoft 365 administrator to open Microsoft 365 admin center → Health → Service health → Exchange Online.
   • Any active incident or advisory will include its status, affected services, and Microsoft’s updates. Administrators can also select Report an issue when no matching incident appears. ^2^
   • If the administrator cannot sign in, the public Microsoft service-status page is listed in the same documentation.
3. Only your mailbox is affected: ask the administrator to check your Exchange Online license:
   • Open Microsoft 365 admin center → Users → Active users.
   • Select your account and open Licenses and apps.
   • Confirm that a valid license containing Exchange Online is assigned and enabled.
   • If it is missing, assign it and allow provisioning to complete. Microsoft states that provisioning normally takes under 30 minutes but can sometimes take up to 24 hours. ^3^

Do not remove a currently valid license merely as a general reset: removing an Exchange Online license can disconnect an ordinary mailbox, and Microsoft documents a 30-day recovery window before permanent deletion in the applicable unlicensed-mailbox scenario. ^4^

Administrator escalation

If Service health is clear, the license is valid, and the error remains after the documented provisioning period, an administrator should open:

Microsoft 365 admin center → Help & support → Contact Support

Include the approximate failure time, affected username, whether other users are affected, and the full error details shown on the Outlook error page—but remove session IDs, tenant information, and other sensitive identifiers from public posts. Microsoft 365 business support requests require an administrator; subscriptions purchased entirely through a partner should be escalated through that partner. ^5^

References

1. how to fix outlook website something went wrong - Microsoft Q&A
2. How to check Microsoft 365 service health
3. Delays in provisioning of user/mailbox or synchronizing changes in Exchange Online
4. Can't access the mailbox of an unlicensed user in Exchange Online
5. Get support for Microsoft 365 for business