After OS patching and reboot of a physical #Linux server used for Icinga2 #monitoring, the Icinga 2 services did not work as expected. Well, actually they did - sort of.
Deeper analysis showed a time sync issue on the rebooted physical server, causing a hiccup in the two #Icinga2 master nodes (running as a cluster).
https://www.claudiokuenzler.com/blog/1476/monitoring-backend-icinga2-not-running-time-sync-issue