When users without a Teams license try to access a customer's Teams tenant via Entra ID guest access, login can silently fail with AADSTS errors. The root cause is disabled service principals in Microsoft Entra. Three enterprise applications must be activated: Microsoft Teams Services, Office 365 SharePoint Online, and Microsoft Teams AuthSvc. Each can be re-enabled via the Entra admin center by toggling 'Enabled for users to sign-in' on the application's Properties page.
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