Okay, thanks to some suggestions from John Rouillard, my problem has been
solved. There were actually two problems, both involving permissions, and
both involving creating the lockfiles.
Problem 1: /opt/majordomo-1.93 wasn't group-writeable by majordom, although
it was owned by majordom. I just didn't read the very first paragraph in
the Installation section of the README file closely enough.
Problem 2: The .archive directory wasn't group-owned and group-writeable by
majordom. The documentation is somewhat less clear about this, but this
seems like a problem that a user could easily trip over. Since they own
the .archive directory, where get'able files live, they can change the
group permissions, possibly screwing up the locking mechanism, and causing
the problem I've been tracking down.
It seems strange to me that both the top-level majordomo directory and the
.archive directory would have lock files. Wouldn't one directory be better?