The files in /etc/ups directory has very restrictive access rights:

-r--r--r--    1 root     root         3730 Feb 10 10:10 ups.conf
-r--r--r--    1 root     root         1495 Feb 10 09:52 upsd.conf
-r--------    1 root     root         2303 Feb 10 10:02 upsd.users
-r--------    1 nut      nut         11109 Feb 10 10:03 upsmon.conf
-r--------    1 nut      nut          3884 Feb 10 09:52 upssched.conf

Read-only for user nut is Ok as this user is not intended to modify
these files.

Please set write access for root for ups.conf, upsd.conf and upsd.users.
This allows easy edit with no error messages (root is always able to
write to these files anyway).