It was working - I had replied to one or two posts in the same session and then... always the above error message regardless of whether I tried "quick reply" or "advanced reply".
However, as I am now replying to this, it seems to be working again. Curiouser and curiouser.
[Sorry I don't recall which of several posts I tried to reply to yesterday and couldn't, although I believe I did try replying to this one and also couldn't - content of my reply would have been "It's now working!"]