Hi Folks,
Operating on the assumption that I was fighting with "mod_rewrite", I looked at /.htaccess.
There was a helpful comment about comprehensive rewrite rules in /zp-core/rewrite-rules.txt, and there I found a section labeled rewrite rules "created by plugins". I thought that might be my problem, since I have only three plugins active at the moment, so I comment those rules out to no effect.
Next, I copied the contents of /.htaccess to httpd.conf and that fixed my problem, but I'm not at all happy about this. I don't understand why the exact same code in httpd.conf will not work in /.htaccess, so needless to say, I still don't understand my problem.
Does anybody have any observations?