The difficulty with the online forms, including the Locus Poll form and the subscription form, has been resolved. (The problem was that the emails to locusmag.com generated by submitting these forms weren't being received; they were disappearing somewhere.) The problem does seem to relate to some recent e-mail upgrade by Locus Online's hosting service, CI Host. This is what they said:
We have been upgrading our email programs on our servers and in this case the nobody user which the webserver runs as did not have the correct permissions. Our admin fixed this once we brought it to his attention.
Now why didn't I think of that.