Most likely caused by "Memory soft/hard limit" for lsphp5 external app, increase the limit, or reduce the cache memory size for APC.
http://www.litespeedtech.com/docs/webserver/config/extapps/#memSoftLimit
Please send more information to bug (AT) litespeed ... ,
we need a copy of the httpd.conf and all configuration files included to reproduce the problem in our lab.
How about trying a regular CGI script which print the output of "id"?
Maybe try
strace -f -p <pid_of_lshttpd>
it will show what happened when lshttpd tries to start PHP in suEXEC mode.
It is not related.
Are you using a real group for the Force GID?
looks like syscall "setgroups(1, &gid);" failed for some reason.
Which OS are you using?