The documentation no longer mentions "config_backend" in the global configuration (see https://icinga.com/docs/icinga-web/2.11/doc/03-Configuration/#global-configuration) and the changelog mentions that "user preferences" are now stored in the DB (see https://icinga.com/docs/icinga-web/latest/doc/80-Upgrading/#upgrading-to-icinga-web-211x) though from the tracking bug it is not immediately clear what goes into the ini files and what is stored in the DB (see https://github.com/Icinga/icingaweb2/issues/3770).
Debugging Icinga\Module\Monitoring\Backend\MonitoringBackend:loadConfig() confirms that the backends ($backends) configuration is actually empty, i.e. that no backends are defined in the configuration (see https://github.com/Icinga/icingaweb2/blob/v2.11.4/modules/monitoring/library/Monitoring/Backend/MonitoringBackend.php#L160 for context). -- Guido Berhoerster
