Sometimes PHP websites return Error 503 page when LiteSpeed is used; however, users are not reaching LVE EP limits.
The issue may be caused by CSF/LFD killing PHP processes. If you have it installed, then check the log:
grep -i kill /var/log/lfd.log
To resolve the issue for good, the /etc/csf/csf.pignore file should be modified - add the processes to be ignored:
echo "exe:/usr/local/lsws/bin/lshttpd.5.1.13" >> /etc/csf/csf.pignore echo "exe:/usr/local/bin/lsphp" >> /etc/csf/csf.pignore
And restart LFD service:
service lfd restart
More information in official CSF documentation, Chapter 8 of https://download.configserver.com/csf/readme.txt