RDP gives the message Internal Error has occurred when attempting to connect to 1 server (2012 R2) from any Windows computer in the network.
Step I: On the server in question go to C:\Program Data\Microsoft\Crypto\RSA
Step II: Rename the MachineKeys folder in that directory.
Step III: Restart the Remote Desktop Services and Remote Desktop Configuration services.
That should re-create the Machinekeys folder.