I changed the site setting for
“tag style” (UPDATE: oops, no, it was actually “tags sort alphabetically”) which seemed fairly innocent. However after doing so, the front page of my site is blank for everyone, with or without an active session. (Even after changing the site setting back.) This is also true for pages like /latest, /categories, etc., but not a problem for individual topic pages or other non-topic-listing pages on the site.
Resetting tags sort alphabetically to default (off) restored the pages.