1) everyone authenticates using their domain credentials; at their PC as well as thru View.
2) As far as the agent is concerned, 5.3 is installed.
3) I see the active session in View Admin - that is the CONSOLE session.
Per the User, connecting works fine for a few days then he will get this error and is forced to connect to the PC another way and log off from the console before he can connect via View.