Norton Ghost 15.0 stops at 5% - "unable to connect to the agent"

Norton Ghost has been performing well for me for years on this (Windows 7) machine and its predecessors. It still works perfectly on my  wife's Windows 7 PC.

Suddenly 2 months ago (last clean backup November 27) it stopped working. When backing up, the progress meter stops at 5%. There is no warning notice that anything is wrong, but when I look at the "hidden icon" at the bottom right-hand corner of the screen, the Ghost logo has a diagonal red line through it and a notice "Disconnected. Unable to connect to the agent. Service not running."

Reading recommendations in reply to similar posts in the past, I deleted and reinstalled the product. The problem is still there.

I can't think of anything new I installed on the machine, or any other changes I made at about the time Ghost stopped working.

Any suggestions?