Organizing and updating events, classes, and meetings within the Teams calendar
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You're encountering a known error code 500 in Microsoft Teams, specifically when trying to access your Calendar. This error, despite your M365 Business Premium license, strongly indicates a problem with your Exchange Online mailbox status, not necessarily your Teams license itself.
May I double confirm that you checked with Admin if your account still has License?
To check this kindly sign into 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.
Moreover, please try clearing Teams cache and let me know if there are any changes: Clear Teams cache - Microsoft Teams | Microsoft Learn
If the issue persists, I'd recommend you submit a support ticket. Kindly ask your admin 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.
I 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.
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