Mod Security Log Files Empty

Hi,
i have noticed that my mod security Log Files are empty.
Nothing happens there.

I restartet, stop & started, disabled, reenabled, changed Modus - nothing.

System:
Plesk Onyx 17.5.3
Debian 8.9

And via SSH it sais, that Service is running.

Any Ideas?
Could it be something with Cron Job?