I agree with Alan's suggestion. If this doesn't work, I would also check to make sure your set top box is set to 1080i and not 1080p; even though the Slingbox Solo doesn't transmit High Definition, this can still cause issues.
You may also see this issue if you are running an HDMI cable from your cablebox to your TV, as this error does tend to pop up as a symptom of HDCP/SOC. I'll link an article below that goes over HDCP/SOC, just in case you're not familiar with those copyright protections. The article is in reference to the Slingbox 500, but the information is the same.
I hope this helps!!