Based off what was described it appears that you may be running into an issue with either the resolution or FPS (Frames Per Second) your cable box is currently set at. We commonly see this issue when an cable box is set to higher than 30 FPS as the Slingbox M1 has certain FPS that it does not support. Please see the article below for an more in depth analysis:
never had the problem of unsupported signal, ever. Same cable box and nothing changed for 2 years with my sling 350 box. Updated software today, forced by sling media, now my slingbox has "unsupported signal". This is ridiculous.
I was a huge slingbox fan, now I am really offended by this move.
This should be illegal.