/opt/rudder/etc/hooks.d/policy-generation-finishedto stop. You can have more detailed logs about hooks by changing log level in /opt/rudder/etc/logback.xml, the part about hooks:
<logger name="hooks" level="info"<- use
trace(which is very very verbose)
/var/log/rudder/webapp/2020_09_22.stderrout.log, there wasn't anything usefull? I suspect the second, I believe we don't see max open files limit from the app. How did you find out ?