Hi @Mark Bewley,
Good day to you! Welcome to Microsoft Q&A forum.
To understand the situation and be able to offer you relevant suggestions, we would need a little more information from you. Kindly help us in providing the following information.
May I double confirm that you checked with Admin if your account still has License? To check this kindly sing into https://admin.microsoft.com/ with Admin credentials--> choose Active User under Users-->click on your username--> choose Licenses and apps-->Expand Apps--> Make sure that Exchange Online(Plan1 or 2) is selected. If your account has License kindly uncheck it and reapply it then wait the sync period to see if it will make any difference
If you don't have Admin privilege, kindly contact your organization Admin to check above for you.
After performing license checks or role adjustments, it’s also important to clear your browser cache, as outdated session data may interfere with updates.
You may also refer to Getting a mailbox not found error in Outlook on the web - Microsoft 365 admin | Microsoft Learn.
If your user account is assigned too many admin roles, it may cause unexpected behavior. To check this:
- In the Admin Center, go to Users > Active users.
- Click your username > scroll to Roles > click Manage roles.
- Remove all roles except Global Administrator, if applicable.
- Click Save changes and try accessing again after clearing your browser cache.
If the issue still present, you may also (use admin account to) go to Office 365 Admin Center-> Support-> New Service Request. The support engineers there have the correct escalation channel, and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket: Ways to contact support for business products - Admin help. The engineers in related team have higher permission and resources than us to help check the root cause from background directly.
We look forward to your response. Kindly note that the above requested information is required for further analysis and troubleshooting of your situation. Your patience and cooperation are much appreciated. Have a nice day!
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