After installing the usual client security updates on my Microsoft Windows Client Virtual Machine I was suddenly no longer able to connect to the customers Windows Servers. First, I had suspected a server security update shutting out my VM (which would have been understandable). But further investigation pointed to a client issue. Especially the specific error message (if you read it *ahem*) helps:

An authentication error has occurred.
The function requested is not supported.
Remote computer:
This could be due to CredSSP encryption oracle remediation.
For more information, see https://go.microsoft.com/fwlink/?linkid=866660

So if you read upon that link, it appears that a Windows Update on my client installed and activated the CredSSP fix for CVE-2018-0886 in "Force Updated Clients" mode which hindered me to connect to the unpached (*ahem*) servers.

In order to "fix" my access problem, I had to modify the behaviour of the client fix via this command line:

reg add "HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters" /f /v AllowEncryptionOracle /t REG_DWORD /d 2

/via Remote desktop connection error after updating Windows 2018/05/08 - CredSSP updates for CVE-2018-0886 - Super User