I've set up WP multisite, done all required modifications in both wp-config and .htaccess files. Now when I try to create a new site in the network I get an e-mail notification in my admin account email inbox however Wordpress does not copy any files to new location. The path is empty (except for index.html automatically created by DA). Additionally, when I try to access the back end I get an Error 500 instead of 404.

The website is old enough to have only the subdomain option available. The subdomain has been created in DirectAdmin. It has the same access privileges (755) as other directories in my current WP installation. I can upload files there via FTP and everything works fine. Also when I enter the subdomain directory (www.domain.com/subdomain ) in my browser I get redirected to the subdomain (subdomain.domain.com) which is correct behavior.

So everything seems to work perfectly in theory but practice shows the opposite. Is it possible that I've damaged some rewrite instructions in .htaccess file? Or may it be a different cause?

I'd love to receive a bit of help. The WP Codex doesn't provide any answers. Neither does DA documentation. There are some threads over the Internet but they are mostly about establishing a network, not much more.