Martin, as you are running a later system, perhaps you are hitting bug 1497782 - https://bugs.launchpad.net/powernap/+bug/1497782
Do you see any errors in /var/log/powernap.err ?
Martin, as you are running a later system, perhaps you are hitting bug 1497782 - https:/ /bugs.launchpad .net/powernap/ +bug/1497782
Do you see any errors in /var/log/ powernap. err ?