In some cases when trying to Remote Desktop (RDP) a server you’ll get the following error message:
An authentication error has occurred. The function requested is not supported Remote computer: rdpserver.domain.local This could be due to CredSSP encryption oracle remediation. For more information, see https://go.microsoft.com/fwlink/?linkid=866660
The error dialog may look like this:
In most cases this will be due to an Encryption Oracle Remediation policy setting on the client blocking the RDP connection to the server.
The best solution is to update the CredSSP updates for both the client and the server to establish RDP in a secure manner.
However, updating servers may require some planning.
For a quick workaround set the following registry key on the client:
REG ADD HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters\ /v AllowEncryptionOracle /t REG_DWORD /d 2
For further information refer to this article.