Just in case, although you're getting a different error message, I'd try the following document with that one:
(The symptoms of this one can sometimes bleed over into looking like the damaged SC Info folder cases, and vice versa, so it's probably worth having a check.)
I tried it (there was a shared folder, so went down that path), restarted, and no change. I had re-deauthorized my computer in the itunes store at some point in this problem and can't authorize it again - don't know if that's part of the problem or not. (Comes back with We could not complete your itunes store request error -42408.) In the meantime I find I have no problem with the other parts of itunes; guess I'll buy my music from elsewhere, and I had saved my Apple store music to CD previously, so I just reloaded it and I can play it. I just like everything to work like it should! Many thanks for your suggestion.