"Modern authentication failed here, but you'll still be able to sign in. Your status code is 4c7" error in Microsoft Teams

You can't sign in to Microsoft Teams when you connect from a corporate network where AD FS is installed. Additionally, you receive the following authentication error message:

This error occurs because Microsoft Teams requires Forms Authentication. When you deploy AD FS, Forms Authentication isn't enabled for the intranet by default.

If Windows Integrated Authentication fails, you're prompted to sign in by using Forms Authentication.

To resolve this issue, enable Forms Authentication by using the AD FS Microsoft Management Console (MMC) snap-in on the computer that has the local copy of Active Directory. Here's how to do this:  

1. In the navigation pane, browse to Authentication Policies.
2. Under Actions in the details pane, select Edit Global Primary Authentication.
3. On the Intranet tab, select Forms Authentication.
4. Select OK (or Apply).

Here's how to Configure Authentication Policies.

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