I can't register account on tigase.org through Jabber/XMPP service, is it a bug?
Yes, you can't and no, this is not a bug.
This functionality was intentionally disabled in Jabber/XMPP service on tigase.org because all account management, including creating new user account is handled through the web site.
If you wish to try tigase.org service you can register new account on registration page and then you can connect to Jabber service straight away after registration is complete.
Please note we no longer offer public accounts. Accounts in tigase.org domain are available to people involved in Tigase project development only. Everybody is welcomed to join us and you can use the contact form to send your request for an account if you want to work with us.