Hi, Phil and Casey: My problems turned out to be a problem with several versions of IE (5.0, 5.01 etc.).
When I upgraded to the latest 5.5 edition, the problem disappeared.
Of course, it makes me a bit nervous about Zope.. (I'm less worried about the fact that it doesn't work on some of these popular versions of browsers than the fact that when the error occurred there was NO information about what was wrong printed out to the console etc. I spent hours trying to change this or that.. and it was only by chance I decided to try nn 4.5 that I had lying around, and was pleasantly surprised when it worked!
This is particularly annoying from our perspective because that version of IE is just plain *broken*, and working around the problem is nontrivial. But "it's Microsoft" so...