I have found the reason of this problem, but I'm not sure completely. variable $_GET['qa-rewrite'] goes wrong with urls consist of some linux commands on my server.
I think that it might be sever security configuration, but I'm not sure because I don't know where qa-rewrite is set. Is it set in apache? or Q2A source code?
Finally I found the problem.
.htaccess consists of these 3 lines of code
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ index.php?qa-rewrite=$0&%{QUERY_STRING} [L]
Then, os.qanet.ir/13/mkdir makes an incorrect $_GET['qa-rewrite'] variable. But now, I don't know how can fix that.