black screen with cursor on user log out yosemite

I am running OS X Yosemite with current updates and Server app. This happens occassionally.

When I log out of a user, after a major app crash like iTunes crash, I get a black screen with cursor. The computer will go into sleep mode if you wait the required time and when woken is back at the black screen with cursor.

The server, still serves you just cant do anything with the workstation becuase there is only a black screen with cursor. The only way to get a logon screen again is to hard kill the computer and reboot.