I've just found what I think was causing my posting problems. I made almost 15 attempts to post or comment to the forum. Only one or two succeeded. Turns out those were written in Firefox. Those that failed were in Safari. (I'm on a Mac).
Clearly there's a Safari-related bug in Multiply.
(Course it also doesn't help that this -- effectively a mailing forum -- is called a "blog" in the multiply system)
(by thomsk for everyone)