You can get this error while trying to open an RDP session.
An authentication error has occurred.
The function requested is not supported
The reason
This is due to a combination of three factors:
You activated NLA on your target computer
The target computer is not patched for CVE-2018-0886
You enforced the Force updated clients or Mitigated parameters on the source computer
Actually, NLA uses CredSPP (for pre-authentication) which is impacted by CVE-2018-0886.
The most common scenario is the following one:
You enforced NLA on your servers since a long time
You recently patched your workstations for CVE-2018-0886 and enforced the Force updated clients or Mitigated parameters on your workstations
However, meanwhile, you didn’t patch your servers for CVE-2018-0886
An now you are stuck with this error message when you try to open an RDP session on a non-patched server.
The solution
Choose one of the following solutions and see links in the More about section below if you don’t know how to do it.
Patch your target computer for CVE-2018-0886 (Recommended)
Enforce the Vulnerable parameter on the source computer (Not recommended)
Disable NLA on your target computer (Not recommended)
Despite solution 2 and 3 are really not recommended, they can be a temporary workaround until you patch your target computer.