>> Here it works with jabber.freenet.de.

> How did you register an account on that server?

I created my account there with Jabberwocky about a decade ago. It was possible to register the account just as described in the Jabberwocky.guide.

> It doesn't work here with Jabberwocky.

So they seem to have changed something. You should be able to register the account with any other popular Jabber/XMPP client, though.