udev rules are applied too late (priority 52), resulting in nut privileges being overwritten.
The final result is that the driver doesn't start, and connection failure to the driver / UPS are reported by upsd and upsmon.

The fix is simply to set priority to 62 by renaming the rules file:
$ mv /lib/udev/rules.d/{5,6}2-nut-usbups.rules

This bug affects:

• Debian: http://bugs.debian.org/721600
• Fedora / Redhat: https://bugzilla.redhat.com/show_bug.cgi?id=488368
• Ubuntu: https://bugs.launchpad.net/debian/+source/nut/+bug/1099947
  and probably all Linux distribution