My standard CHMOD is 755. On my pages in every directory I have empty index.php file as default, here no. What chmod will be best to prevent directory listing without errors in q&a? No info on install page http://www.question2answer.org/install.php
This is actually done by the server, nothing to do with Q2A. You should be able to add the following to your htaccess (at the beginning of the file) to prevent directory listings:
Options -Indexes
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