If you are getting an error using RDP to connect to a Windows Server and error say “Remote Desktop Connection: An authentication error has occurred. The function requested is not supported. This could be due to CredSSP encryption oracle remediation”, this is because you are connected from an unpatched client to a patched server or a patched client to an unpatched server.
To fix this issue, install the May 2018 Windows Updates on both the server and the local PCs.
Microsoft has been patching a vulnerability in RDP/CREDSSP with the patches released this month (May 2018) and previous month or two, and as of the May 8 updates, it requires BOTH the client PCs and the Windows Server to both have the May patches installed.
Below is a link about it but best is to apply the patches….
https://blogs.technet.microsoft.com/yongrhee/2018/05/09/after-may-2018-security-update-rdp-an-authentication-error-occurred-this-could-be-due-to-credssp-encryption-oracle-remediation/
UPDATED 5/10/2018 –
Additional links discussing this issue below. Best approach is just install Windows Updates on both Servers & Client/local PCs and everything should work. If you are unable to patch your server immediately, there are some suggested workarounds (registy/GPO modifications, disable NLA (not recommended due to lower security), etc. We strongly recommend you apply the May 2018 Windows Updates.
https://blogs.technet.microsoft.com/askpfeplat/2018/05/07/credssp-rdp-and-raven/