I resolved it. Posting the fix for anyone else hitting this after updating to 2026.8.
Root cause: Our site (Communiteq-hosted) {EDIT by OP/RS later - I WAS WRONG; site hosted by Discourse 
} updated to Discourse 2026.8.0-latest.1, which bundles Calendar and Events into core. After the update, the Events plugin showed as Enabled at admin/plugins, but the discourse post event enabled site setting was off. This caused a server-side 404 on /discourse-post-event/events.json and the “page doesn’t exist” error on /upcoming-events for all users.
Fix: Re-enable discourse post event enabled in site settings. Calendar and all events repopulated immediately. No data loss since events live in topic posts.
Flagging for the team: the enabled state of this setting may not be carrying over during the core-bundling migration. Worth checking whether other sites updating to 2026.8 will hit the same thing.