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:
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.