Thanks for reaching out to us in the land of the forums. We do apologize that you are getting this error. The main reason why you are getting this error is due to the fact that you are using a HDMI cable somewhere in the mix of your connections between, the TV, the Slingbox and your cablebox. We have a great article that will explain more about these HDMI cables, and why they may not work with your set up. It will also include a fix, which is simply switching to component cables.
Hope this helps!
The Slingbox Moderation Team