That error simply means that cable box can not communicate back to the comcast servers. If it has anything to do with the sling its that your cable box is connected behind the slingbox. If your cable is connected from the wall to the sling then to the cable box the sling prevents signals going back to the cable server.
If that in not the case. The error is still the box can't communicate back to the comcast servers. It could be a defective box or simply not enuff signal to reach.