Please try the latest build of 5.0.13 or 5.1.3, the cache folder should be cleaned everyday.
/usr/local/lsws/admin/misc/lsup.sh -f -v 5.0.13
or
/usr/local/lsws/admin/misc/lsup.sh -f -v 5.1.3
Please try the latest 5.0.13, which should block attack passed by front-end proxy. Right now 403 error returns and IP added to black list.
/usr/local/lsws/admin/misc/lsup.sh -f -v 5.0.13
Anyway, you just need to increase "Process soft limit" for lsphp external app.
https://www.litespeedtech.com/docs/webserver/config/extapps/lsapi#procSoftLimit
You should run "ulimit -a" from a php script, to check the real value of "max user processes" applied. It is too low. Need to raise process limit for lsphp external app.