I can confirm that upgrading from 3.5.1.dfsg-2.1ubuntu1.1 to 3.5.1.dfsg-2.1ubuntu1.3 (from ppa:ubuntu-security-proposed/ppa) fixes this issue on xenial.
Installing ppa:ubuntu-security-proposed/ppa's 3.5.1.dfsg-2.1ubuntu1.3 on xenial without a previous nagios3 installed is OK too.
I can confirm that upgrading from 3.5.1.dfsg- 2.1ubuntu1. 1 to 3.5.1.dfsg- 2.1ubuntu1. 3 (from ppa:ubuntu- security- proposed/ ppa) fixes this issue on xenial.
Installing ppa:ubuntu- security- proposed/ ppa's 3.5.1.dfsg- 2.1ubuntu1. 3 on xenial without a previous nagios3 installed is OK too.