No entries in grav.log

while trying to investigate some other problem i recogniced, that my ./logs/grav.log is not growing.
following an other thread i already checked space on the corresponding partitian and set all permissions (just for testing purposes). In system.yaml i set errors.log:true
is there anything else, that i have to check?

The grav is running on a linux behind a ReverseProxy (Apache 2.2)

did anyone else experience this?