404 error on single portfolio pages

Symptom

Projects or the Portfolio page return Page not found.

Cause

Rewrite rules went stale after a slug or a permalink change, or the project base collides with a page or a category that has the same slug.

Fix

1. Open Settings → Permalinks and click Save Changes. This rebuilds the rewrite rules; you do not have to change anything on the screen.
2. If the error stays, check that no page other than the Portfolio page uses the slug portfolio.
3. Check that the category base in Settings → Permalinks differs from the project base.
4. Deactivating and reactivating the plugin also flushes the rules.

On the Avada theme the cause is different; see Avada theme Portfolio post type conflict.