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A cache belongs in Nextcloud's memory cache rather than in the app config. Use the distributed cache (e.g. Redis) with a one-week TTL when one is available; Nextcloud falls back to the local cache (APCu) when only that is configured. Without any memory cache the lazy app config value stays as the fallback. A leftover app config value is removed when the categories are stored in the memory cache, and saving the settings clears both.