Thank you very much @sean for your quick help. I really appreciate it.
The site is now working (partially), and there are some possibly strange things going on that could give you a clue about what is the problem.
I can access the admin site only by adding /wp-login.php/ to the end of sites URL (this should be accomplished by adding /wp-admin/ also). Then, I get redirected to the admin under the original domain which is still pointing to WF server (maybe this problem is connected to the fact that I still have original domains not migrated?).
However, when I try to add only /wp-admin/ or anything else arbitrary, like /login/, /admin/ or /whatever/ to the arbormagna.opalstacked.com subdomain, it doesn't give me 404, but rather stays on index page but with "URL+/whatever/" in address bar (e.g. http://arbormagna.opalstacked.com/admin/whatever/).
I am not much in WP or PHP, just using it for the "lite" sites, so this behavior is really strange to me, but I suspect it has to do with redirections and/or maybe with original domains?