Yes, I included my files, at the time of the error. I've since rebuilt the entire menu, following yet another reinstall, although I simply couldn't uninstall CSSMenuWriter at all, it just wouldn't let me.
Anyway, I completely reinstalled over the top, this being about the 4th time I've done that, and, touch wood, the error isn't back yet.
I am finding CSS MenuWriter to be quite unreliable, at the moment. I'm hoping that some clarification on what that error is will pin-point what my problem has been.