Dear Jimp,
many thanks for your hint.
Erasing all logs did the job, so this one is currently solved.
To further avoid a log stall, do you know of a method to do so?
In particular, would a syslog server like syslog-ng do the job?
I will have post another question on the release depends here: https://forum.pfsense.org/index.php?topic=74897.0