Zen Cart v1.3.8 is much less prone to mod_security blocks during install-time than prior versions were.
The /zc_install/.htaccess file may be no longer needed, and might be removed in v1.4 to help avoid this issue.
I too am getting this 500 after a successful install. I try access the web page for my zen cart and I get 500 error.
I deleted the htacess and zc install folder and still nothing. I am so new at this I could scream.