According to https://community.icinga.com/t/icinga2-2-11-4-1-nagstamon-getting-typeerror-with-new-host-objects/4761 the Icinga version 2.11.4.1 causes trouble for Nagstamon. This is solved in the latest testing release of Nagstamon available at /download/#testing.
Icinga 2.11.4.1 problem solved with Nagstamon testing release 3.5-20200731