Common causes for 500s include:
1) Permissions. Files and directories must NOT be world-writable (aka NOT 777 or 666). The recommended permissions are 755 for directories and 644 for files
2) .htaccess. Check all of your .htaccess files and remove anything a la php_flag/php_value, etc. as these don't work on Free Hosting. Also check if you are using mod_rewrite (RewriteRule) and add a RewriteBase directive for that