Got a reply to my ticket. Turns out "you cannot set those settings", so no one has been able to post for over a year. (Obviously one *can* set a forum that way; it just doesn't work). I have modified the settings so that anyone can post or reply, but the message will not appear until a moderator has approved it (at least in theory). I will also add the code back in for the reply button.

Quite a coincidence; I add the code, and two (count 'em, two) new members try to post to that forum within about a day.