It seems recently many users are receiving an error logging into Remote Desktop Servers (Terminal Servers) from off-site. The error reads:
An authentication error has occurred.
The function request is not supported.
Remote computer <ServerName>
This could be due to CredSSP encryption oracle remediation.
For more information, see https://go.microsoft.com/fwlink/?linkid=866660
This is a result of a March 13th update. The previous error message was shorter, but an Apr 17 update elaborated the error message to read as above.
The link explains how to resolve using group policy but the simple fix, as of May 8th, is to apply the KM4103725 monthly rollup update. This will require a reboot on most servers, but should resolve the problem once complete.