When trying to establish Remote Desktop(RDP) connection to a server or computer, you get error “An authentication error has occurred. The function requested is not supported.”
Microsoft has released an update on March 2018 to fix an issue with CredSSP, “Remote Code Execution” vulnerability (CVE-2018-0886) which affect RDP connections. If one of the following updates are installed on the client or server, the RDP connection attempt will fail with this error.
Windows 10: KB4103716, KB4088779, KB4103723, KB4103731, KB4103727, KB4103721, KB4103716
Windows 8.1: KB4103725, KB4103715
Windows 7: KB4103718, KB4103712
Windows Server 2008: KB4056564 Windows Server 2008 R2: KB4103718, KB4103712
Windows Server 2012: KB4103730, KB4103726 Windows Server 2012 R2: KB4103725, KB4103715
Ensure both client & server side have above patch installed so that RDP can be established in a secure way.
The update can be downloaded for each operating system here: https://portal.msrc.microsoft.com/en-us/security-guidance/advisory/CVE-2018-0886
Change the following registry key to allow unsecured RDP connection