Channel changing stopped working

I have a Series 2 DVR with FIOS-TV from Verizon. Suddenly, the DVR cannot change channels anymore. I am using the serial port to connect to the Motorola STB and have had no problem for months. Then on Monday evening I noticed that the channel changing is broken. The DVR cannot change channel and the Tivo remote control also cannot change channels. I am seeking a clue as to where to start looking for the problem.

I've checked the serial cable and is well-seated. My main suspect is...

Channel changing stopped working

Channel changing stopped working