Bug in json-c library
After installing the latest security patches on your system Nagios and Naemon will most likely not work anymore. The Statusengine Event Broker which gets loaded into the monitoring core depends on the json-c library.
Unfortunately it seems like the patches that should fix security issues CVE-2020-12762 within json-c are causing the problem.
For now the easiest method is to downgrade to the old version of json-c to get your system up and running again.
As far as we know this bug only affects Ubuntu users.