I forgot to add that, before displaying the empty screen, it does go through the "Connected to musicbrainz/freedb server" ritual, but only (I think) once, just after Express Rip has loaded.
I've just noticed that selecting any link from within the <Audio> tab of the NCH Toolbox doesn't do anything, but since I haven't tried this before, I don't know if that's connected to the problem.
Help!
Thanks again.