I just noticed, that the firewall stopped logging after I upgraded on of our firewalls to the latest 20.1 version (OPNsense 20.1.9_1-amd64) some weeks ago.
Neither the live view nor the plain view show new log data, only data from before the upgrade.
Any ideas what how to debug the issue appreciated :)
investigating further, I found that syslog is broken in the latest 20.1 version
https://github.com/opnsense/src/issues/49
... and that 20.7 solves the issue, so that's what I will do :)