Multi-session Windows 10 Enterprise Virtual Desktop (EVD) behaves like a "Single Session" Windows 10 Enterprise
If a Multiple Activation Key (MAK) is erroneously applied to a multi-session Windows 10 EVD, deployed from Azure Marketplace, the OS does not allow multiple users to connect at the same time. This can be repaired by resetting to Key Management Service (KMS) activation with the proper Windows 10 EVD key.
Note: These commands cannot convert a single-session Windows 10 Enterprise OS to multi-session. This solution can only be used to repair what originally was a multi-session Windows 10 EVD OS.
To convert a single-session Windows 10 Enterprise OS to multi-session:
In the EVD, open the command prompt as admin.
Run the following commands:
slmgr /ipk <KmsKey>
slmgr /skms kms.core.windows.net