This is usually caused by a resolution/refresh rate combination that is not supported by the Slingbox. You can fix this by changing the video resolution (in the cable box settings) to 1080i. You will still get HD video and will keep you from trying to "distort" the video at the cable level.
Hope this helps.
Sling Media Support
I have been having a problem feeding component video into a Slingbox Solo. This works fine until I display a menu and then I get the message “unsupported signal like 1080P is connected” or “Weak or no video detected”
It would appear that the Slingbox wrongly interprets the continual changing between the text and background at a digital signal.
This is detected on the Y input as the Pb and Pr leads can be disconnected and the problem persists.
The solution is to distort the Y input sufficiently that it no longer looks like a digital signal but not enough that it degrades the video content. This is achieved by connecting a low value capacitor directly across the Y input.
I found that 1000pf cured the problem on one Slingbox whilst I had to go for a slightly higher value on another. I could not detect any obvious degradation of the video. The trick is to use the lowest value capacitor that cures the problem.