update: a user still experience 500 error after following above instruction.
when run addon_lsws.cgi directly on command line:
-bash: /usr/local/cpanel/whostmgr/docroot/cgi/addon_lsws.cgi: /usr/local/cpanel/3rdparty/bin/perl^M: bad interpreter: No such file or directory
so the addon_lsws.cgi is in MS-DOS text format!
500 error finally gone and WHM lsws plugin installed with success