Resolved (kind of) — Deleting the plugin, then making a new post somehow fixes the feed. No idea why. You have to make a new post for it to work.
An alternative work around if this still doesn’t work is to install a Feedburner plugin and redirect the feed page using htaccess.
Update: It appears the problem is caused by the plugin Maintenance Mode — it breaks all feeds as soon as you change the maintenance status *permanently*, so changing the settings, deactivating/removing/reinstalling the plugin doesn’t fix.
If anyone has a solution or idea of what is being affected, let me know.
