You need to run the Configure Video Inputs and do a scan so it will pick up that your Cable Box is transmitting on Ch3 (or Ch4). Unlike the TV, which I assume you have tuned to Ch3/4, at the moment the Slingbox has no idea what channel to tune to so is bound to show "No Video".
Later: Actually as I remember it will ask you what your setup is and when you tell it you are using a Cable box it will switch to Ch3 automaticaly.
Thanks for your responses. I have tried to configure video inputs several (but not every) time(s). Again no video signal, although on one or two occasions the error message went away and and the small screen showed white.............
One thing I haven't done is tried a pass through to the second TV. That might provide a clue, but I admit I'm totally flummoxed.