Everytime I see it,..it is because the sessions has gotten corrupted in some way. If you were using a real Terminal Server instead of the users RDP'ing into their own workstations you wuld just go into the TS Manager MMC and reset their session. Since this is a workstation you will probably have to restart the workstation.
You REALLY need to be doing this with a Terminal Server instead of workstations so it is actually "managable".