I hate to raise the dead here, but I had this same issue after switching to NUT to control other network devices without spending $400 on an SNMP card to the UPS.
I had "Start APC UPS daemon:" set to No, yet the service continued to run, which caused issues with my UPS, with both the build-in UPS daemon running alongside NUT.
In case anyone else is an unpaid noob, clicking the "Default" button under UPS Settings resets all the settings and stops the APC UPS daemon, and I haven't had any issues since.