Thanks for looking at this so speedily Christian!
Actually, I realised the description was not quite correct. The original error I was experiencing was not when *replying*, but adding an *additional* comment to the paragraph. This error seems to still be happening. It happens when both logged in and not logged in.
(When testing a reply, initially I actually got an ‘Ajax’ error. Although I can’t replicate that (so I can’t repeat exactly what the error was) and now it seems to be working fine. I include this just in case it gives a clue as to what is happening).