My installation was done by a couple of very inexperienced system managers (undergraduates). When they put the site url in site.conf, they used double quotes rather than single quotes, as suggested in the comments. When I noticed this and changed it to single quotes, it started working.
Presumably there was some unwanted variable substitution going on inside the url. An excellent lesson for the students!
Thanks very much, Michael. Your hint was right on the money!