This behavior occurs when someone connects to an RDP session, then locks their local machine. Then, if you try to connect to that session over Control, you see the border flashes and the client become non-responsive. Must terminate the Control Client via Task Manager. Restarting the Control Agent service may be required to get anything to show up in the Control Client after the first occurrence (reconnecting to the same server/Control Agent).
Reference ticket #11836937
Was told a fix is underway but unknown ETA.
Customer support service by UserEcho