No error codes on the 20D? Have you tried cleaning the contacts on the lens and camera body, oxidation doesn't necessarily dull the color. If that doesn't work, sounds like a trip to Canon for repair.
I have tried the lens on another camera, and it doesn't work, I mean the lens does not work. Whereas another lens works perfectly on my 20D.
This is the strange thing. The lens has stopped working all of a sudden. Nothing has happened to it. The contacts (on the lens as well as the camera) are still shining. Plus I think it is not a contacts problem, since if I manually focus in AF mode, the camera beeps if focus is achieved. Plus if I change the AF switch on the lens to MF, the indication on the camera changes, which means that the lens is communicating with the camera. Its just that the autofocus is not working.
Has sth like this happened to anyone else? I assume the lens gets its energy from the camera, or is it that there is some hidden battery somewhere in the lens to drive the USM motor?
I never thought something like this could happen to an L series lens.