After upgrading to the latest icinga2 (2.7.2-1.xenial), Icingaweb2 has lost its connection with icinga ("backend not running" message in the web UI). Icinga2 is definitely up and running fine, so something is not right. I then upgraded icingaweb2 too (to 2.4.2-1.xenial); no change.
We have a setup where the db (MySQL), icinga and icingaweb2 are each on a different VM. Icingaweb2 is running on PHP-FPM and Apache.
I've been trying to check all the logs (mysql, icinga, php, apache, system logs) I can think of but can find no errors indicating the reason. Network is fine, and nothing has been changed in the system AFAIK, apart some stock package updates from Ubuntu repo. This system has been in production for months now.
Any ideas where should I be able to see indications of what might be wrong?