Seen this on an isolated machine before but since upgrading to the latest version almost every mac client we have unattended access to is doing this now. In the unattended list, the machine will display "root" as the user and there will be no thumbnail of the screen. If you try to connect it will pop up normally but just sit at a black screen. There will be a few messages sent/received but will not move past a few.
Customer support service by UserEcho