It seems that you are having some issues in order to stream the Slingbox SOLO video signal.
The fact that you have tested the video output with a different TV source and it worked fine means that the Slingbox is working as it is supposed.
Even though you have not provided further details, I recommend you to check the cable box-TV set connection. If you are using the HDMI cable, this could be the reason for this behavior. Keep in mind that the HDMI cable and the Slingbox (any model) are not compatible due to the HDCP (High-bandwidth Digital Content Protection) restrictions.
The following links will provide you with further details about this situation.