I think that it is some kind of SPTI problem. I installed both the trial version of Elaborate Bytes' CloneDVD and VSO Inspector. After installing them, ImgBurn works fine with either the ElbyCDIO or Patin-Couffin interfaces selected in Settings/IO. If I reselect SPTI, it doesn't work again.
So there is something going on with SPTI and x64 on my particular computer, not with Imgburn. Thanks for the assistance.