I mad some changes to the board system that gives us a lot better tracking of IP addresses. In the process of this, one file went... haywire. I don't know why, only one tiny thing changed in it. It's as if there was some version difference... like an I'd uploaded an old file. I have so many versions of this board knocking about, it could have been that.
But I didn't know where the problem was for a while. But solved it by getting one of the files from the source zip file, and then it was okay.