Not only are H.L.'s archive pages not currently working (404 error), for several minutes it wouldn't let me edit a post at all. That problem is now out of the way; but the archive pages are still not working. I can't think of any fiddling with the files that would have caused this; as far as I can remember I haven't messed with the template or the code at all since before I looked at an archive page (for the Philosophers' Carnival), except to add a link on the right sidebar, which shouldn't have done anything. But none of my archive links work - LinkChecker finds them broken. I'll probably go to the WP forums, but I'd like to be able to give them more information than, "My archives links are broken, I don't know why." Does anyone have any guesses at all?
On the plus side, I did finally find a broken link that LinkChecker kept picking up on but I couldn't find before - the calendar's links are weird.
UPDATE: I figured out what it was: the .htaccess file was missing the correct rewrite rules. I have no idea why it was missing them; I put them in when I did the template originally, and it has worked since then. The calendar links still are weird, but I think I should be able to fix that with just a bit of experiment.
UPDATE: Fixed the calendar. On browsing the support forums, it turns out that it's a common problem: the calendar coding assumes that the permalink URLs have a structure that's actually optional (and which I don't use). So a little change in a file fixed the issue.
One thing that still bothers me is that I can't access .htaccess through the WP template. I know I've done it before, and the permissions are right; so I'm not sure what's up with it now. But that's a minor problem.