I just got this rectangular message on my TV screen. Funny thing is that the program guide DOES work. I just can't get rid of the message on my screen that blocks a lot of the picture. Comcast basic support couldn't fix it. Tried turning off cable box, rebooting, etc. No luck.
We had the same rectangle box "Program Guide not responding. Do you want to wait?" Our program guide DID work. Just couldn't get rid of box. Did a system refresh, unplugged box etc. Had to call Comcast. They were frustrated too! Ended up scheduling a tech to come out. I switched inputs to Apple TV then back to cable. Lo & behold rectangle box was gone! Yay! Cancelled appointment. Try switching inputs. Good luck