XRE-00251 on one of three TVs, no help from Comcast

More than two weeks ago I installed our new X1 boxes. The main box and one secondary one initialized properly and work fine. The third one continues to get the XRE-00251 error message that tells me to call to have the device activated. I have called 3 or 4 times and they have not solved the problem. My report was supposedly escalated last week. 
The serial numbers on the boxes match what Comcast has on file, the device getting the error code obviously is getting some communication from cable, I returned our old boxes and they no longer show on our account, and Comcast knows we have 3 TVs (and charges us for them).  

I have a support ticket number now but I don't see any place to followup on that. 

Very frustrating. Has anyone found a way to resolve this? I suspect telling Comcast I no longer want to pay for a 3rd device might get their attention, but that's not what I want to do.