Frano,
* Generate a script which will correct ownerships and permissions under /home/*/public_html
Code:
grep '/home' /etc/passwd | cut -d: -f1 | awk {'print "chown -R /home/"$1"/public_html; chown :nobody /home/"$1"/public_html; chmod -R 755 /home/"$1"/public_html"'} > ~/fix_ownerships.sh
* Run the generated script: "sh ~/fix_ownerships.sh".
If after running the script you're still seeing 500/Internal Server Errors, the issue most likely related "php_flag" or "php_value" entries in the account's .htaccess file. Simply rename the .htaccess or comment out the lines.