Peter, I had the same problem, as did someone else. Here's the bug for it:
https://openacs.org/bugtracker/openacs/bug?filter%2estatus=closed&bug%5fnumber=496
My guess was that I thought I had upgraded to 4.6.1, but I actually hadn't. There is a longer description on this link which describes how I fixed it, and what the problem was (as far as I could tell).