Paperback is powered by the Pinboard API which, unfortunately, often gets bogged down by heavy traffic. Paperback monitors the Pinboard API health and will serve you cached posts if the API is inaccessible.
Paperback checks Pinboard health about every five minutes so not to add to the overload. If you'd like, you can retry sooner.
If always, if you're running into issues, you can email me for support.