Sounds to me like the resolution of the video signal being sent to the Slingbox is not a resolution that the Slingbox supports. If I were getting this message on my Slingbox, what I would do is go to the video resolution settings in the cable box itself, and try switching the video format to 1080i. After that, try viewing your Slingbox, and see if that made a difference.
Here article that lists the video resolutions supported by the Slingbox:
Hope this helps!