Are you planning to do a reset of file owners in all user accounts ?
Maybe if you make an script for a complete 'chown -R username,username /home/username' in every user home directory, the problems with error 500 will end ?
Other Topic:
How about customizing php_values in .htaccess? this will not work now, right?
How can i customize these values for some directorys ?
Thanks in advance and best regards