try to diagnose with log files first
tail -n 200 /usr/local/apache/logs/error_log
tail -n 200 /usr/local/apache/logs/suexec_log
Look for error 500 to narrow down the issue.
/scripts/upcp --force
Possible permission error, I would suspect, you can try the following as well, make sure you have a good working full backup first...
cd /home
find /home/*/public_html -type f -name '*.php' | xargs chmod 0644 --
find /home/*/public_html -type d | xargs chmod 0755 --