RDK30306 Error

Hi - main X1 box on my main TV works fine. All the other smaller boxes on TVs throughout the house send back the RDK 30306 error suggesting the cable is not connected tightly - it is.

I've spent over 4 hours this week on the phone with Comcast support and no one has been able to help. I can watch one out of 5 TVs. 

Help!

@ComcastChe any suggestions?