CredSSP and mstsc authentication gives some error sometimes, after installing some needed updates, but e.g. youir server is not yet up to date.
You can set back a reg entry… this one:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters] "AllowEncryptionOracle"=dword:00000002
On the client, save it to a file with .reg extension, as tekst, and double click add to registry, you may need some rights for this.
You can now use mstsc again to your unpatched server, for a while.
Hope this helps.