CredSSP Encryption Error - An authentication error has occured

Cause:

This error occurs when you attempt to create an insecure RDP connection.

The client has the CredSSP update installed and is trying to connect to an unpatched server
The server has the CredSSP update installed and an unpatched client is trying to connect

Solution:

Installation of the CredSSP-Update.

There are ready-to-use PowerShell scripts available (see link).

In my case, I had to select the script for Server 2012 R2.

# Create a download location
md c:\temp


# Download the KB file
$source = "http://download.windowsupdate.com/d/msdownload/update/software/secu/2018/05/windows8.1-kb4103725-x64_cdf9b5a3be2fd4fc69bc23a617402e69004737d9.msu"
$destination = "c:\temp\windows8.1-kb4103725-x64_cdf9b5a3be2fd4fc69bc23a617402e69004737d9.msu"
$wc = New-Object System.Net.WebClient
$wc.DownloadFile($source,$destination)


# Install the KB
expand -F:* $destination C:\temp\
dism /ONLINE /add-package /packagepath:"c:\temp\Windows8.1-KB4103725-x64.cab"


# Add the vulnerability key to allow unpatched clients
REG ADD "HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters" /v AllowEncryptionOracle /t REG_DWORD /d 2


# Restart the VM to complete the installations/settings
shutdown /r /t 0 /f